The problem is that bans take place when a single Pokemon, move, or ability is what's warping the tier. If there's no threats in the metagame that a suspect test deems broken enough to ban, then I think at that point we should just swallow the pill and accept the metagame we have right now, see where things go with people innovating sets and such. Maybe the metagame's not ideal, but if the metagame is populated with enough threats that match-up is that much of a problem, undesireable or not, the game's not centralized or wrapped around anything in particular enough to ban.
I respect your opinion, but not everybody wants to swallow this pill. This game is created for 4vs4 Doubles (known as VGC), but we try to play 6vs6 Single. It is obvious that this game will be unbalanced and that we should try to fix it. If you would rather sit and "enjoy" this meta I can't help it, but many others like me don't accept that. If there is nothing broken enough to ban and the metagame is still not healthy then the meta itself is broken. Doing nothing doesn't fix that.
The fact that Aegislash offers so much defensive or offensive utility ON TOP of potential to Pursuit Trap is what makes him more effective as a trapper. If I see an Aegislash and a Bisharp on the opposing team, feasible since the latter benefits from Aegislash's presence, that's at least two potential Pursuit trappers on the opposing team that I have to walk on eggshells with in using my Slowbro or Latias. One, both, or neither could trap my Latias, but I need her prepared for, say, Keldeo or Zard Y. The reason they become OP is because Aegislash makes Pursuit trapping easier to pull off, or even just to bluff.
It's not like you can't do that now, you know? I already faced teams with Bisharp + Tyranitar or Tyranitar + Scizor and I had to guess who is the trapper. Pokemon is a game of luck, prediction and guessing and I don't see how Aegislash will change that much. Pursuit-trapping is already easy as it is.
Yes, Aegislash can't check "half the meta" as everyone puts it without recovery. However, Aegislash is only ever going up against 6 of the 50-100 threats in the meta, and for a decent teambuilder, he needs to check 3 things at the absolute worst by himself. Considering the kind of utility he offers beyond natural defensive backbone, I think he can reasonably pull that off, since he checks a lot of Pokemon just by virtue of making them harder to use to their fullest. Say my team is Mega Gardevoir weak, so I slap Aegislash on. Aegislash is working double time because on top of checking Gardevoir that I go up against, he also reduces Gardevoir's effectiveness by existing in the meta such that I'll see fewer for him to deal with in the first place.
3 things can be really hard for it to handle.
252+ SpA Mega Gardevoir Shadow Ball vs. 252 HP / 0 SpD Aegislash-Shield: 162-192 (50 - 59.2%) -- 78.5% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Serperior Leaf Storm vs. 252 HP / 0 SpD Aegislash-Shield: 66-79 (20.3 - 24.3%) -- possible 6HKO after Leftovers recovery
If I may expand my example from earlier, Aegislash might be able to handle one threat, but 2 are already pretty difficult to deal with for it. As you can see, it will be hard pressured enough to just keep Gardevoir in check that Serperior, who is normally checked by Aegislash, just breaks through and gets free boosts on it.
I agree that Gardevoir will be less effective, this is to expect. But it will not go entirely useless, it even has the tools to deal with it.
The problem with detecting Lure Aegislash sets compared to others is because of Aegis' splashability. Normally Lures are put onto a team with something sharing checks/counters to their standard set, so they can be picked out on team preview or from certain EV behaviors because its suspicious to stack a weakness to something so powerful you'd need a Lure for it in the first place.
Aegislash can fit on a team anyway without significantly compounding his weakness to anything, because the closest answers he has (Hippowdon, Mandibuzz, Chesnaught) aren't influential enough to require Lures in their own right the majority of the time. Eliminating these is Aegislash luring them for himself to get one more kill/benefit that he wouldn't otherwise. The other problem is that Aegislash usually only has to change 2 moveslots to completely alter how his set functions. Lure Aegislash isn't easily detectable because there's virtually no team for which there is a better option for its slot, in turn meaning there's no team on which its presence over something else should raise an eyebrow.
Lure-Aegislash stacks weaknesses too, just from a different point of view. That's why it is so special.
You already explained how you can detect a normal lure. So how do you do it with Aegislash?
Aegislash only lures in the mentioned Pokemon, so instead of looking at the weaknesses you should look which Pokemon would give this team the most trouble without considering lure-Aegislash. If your opponent has only one or even no reliable way to handle Mandibuzz, you should expect the lure. You noticed how the team is walled by Hippowdon without any answer to it? Scout for SubToxic. Just because Aegislash is common doesn't mean you can't analyse the team before battling. A normal lure doesn't neccessarily have to stack weakness (otherwise it would become too obvious) but due to the nature of Aegislash he sacrifices all of its good trades just to lure in, so they are easier to detect and have a higher opportunity cost than some people think.
And like I already mentioned, after taking only one hit it becomes clear as day what kind of set it is.
You're underestimating the kind of effect SubToxic can have on the tier. There's virtually no Pokemon in the tier resistant/immune to both Shadow Ball and Toxic bar Bisharp, so Aegislash isn't constrained significantly by lack of coverage. Aegislash functions as a blanket check, so he's forcing switches naturally, giving him turns to Sub. Once he Subs, the handicap of "a Protect that doesn't shield from status" is negated and turned into a more powerful option. Now the opponent runs a serious risk if they predict wrong: on top of not breaking the Sub (meaning Lefties for Aegislash, potential Toxic for them), it could weaken their offenses, making the Sub even harder to break on the subsequent turn. The other thing is that SubToxic's defensive oriented nature means Aegislash spends much more time with his Shield form bulk and is less pressured to King's Shield often. SubToxic was a boon for Stall teams because it took Aegislash's defensive utility even further while adding the ability to spread some of the most effective passive damage means in the game.
Gliscor doesn't need to resist Shadow Ball, it wins anyway.
And you don't have to explain how the SubToxic-set works, I know that. Aegislash is still prone to offensive pressure because it looses 1/4 health while using Sub and has no recovery. It may have some sort of surprise effect on some people, but after seeing it they usually adapt to it during battle and deal with it. I didn't say SubToxic is a bad set, but it is overhyped and not as godlike as some people try to make it. You even mentioned it yourself that it was a great addition to stall, which was the playstyle in which 90% of all SubToxic-sets were used because of their access to Wish.
Excadrill I can grant, but Starmie is dancing around Aegislash if he wants to beat it and Spin. Starmie HAS to get Aegislash on the switch turn with Hydro Pump's Analytic boost, meaning on the turn Aegislash is off the field, he can't spin. If he uses Hydro Pump and the switch wasn't Aegislash, he wasted the Spin Chance. If he spins and the switch is Aegislash, wasted the turn and he now risks Pursuit Trapping or a Shadow Ball. Unlike Excadrill, Starmie doesn't win against Aegislash easily or consistently, so Aegislash does make his job harder, even if it's not to the extent some make it out to be. Also, Starmie's interesting Bulky/Reflect Type set takes a hit since his new typing is still weak to both Shadow Ball and Pursuit while lacking the power to beat Aegislash in any situation. And Blastoise was barely viable during Aegislash's era, only to now be met with even more power creep and Mega Slot competition.
And Aegislash was important for spinblocking because he also indirectly made Defog much harder to use. Aegislash beat 2-3 of the most common Defog users (Latis and Skarmory)
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 4 SpD Skarmory: 213-252 (63.7 - 75.4%) -- guaranteed 2HKO
The only particularly common Defogger was Mandibuzz (though Zapdos might pick up as well), who could also take Aegislash on. But what else got popular in an Aegislash meta? Bisharp, a mon that penalizes Defog as much if not more than Spinblocking. Whereas a spinblocker eats momentum, if Bisharp gets Defogged, he is in a position to give you more trouble than if he'd switched in dry. Bisharp's greater prominence made Defogging harder to do, while Aegislash made spinning tricky. Excadrill became the premier hazard removal besides Mandibuzz because he was the only one that could beat both of these two "anti-removal" mons. And that ease of keeping hazards intact is another way Aegislash supports teams, in this case that being Hyper Offense alongside Bisharp, and hazard stacking balance as well.
Starmie will definitely get hurt by Aegislash's presence, nobody denies that. I just wanted to say that it doesn't make it unviable, which you admitted, so we agree on this one.
I mentioned Blastoise because everybody was complaining how it spinblocks everything while only Tentacruel really suffers from it. There are not many viable spinners anyway and besides this one Pokemon every other can get past Aegislash regardless.
Banning KS doen't make sense imo.
Greninja was banned from OU because Protean made him broken. The other Protean user, Kecleon, has no impact on the metagame. So why did Smogon ban Greninja instead Protean ? Because Protean isn't a broken ability, Protean is a broken ability on Greninja.
Now let's apply this reasoning to Aegislash and King's Shield. KS makes Aegislash borderline broken and forces 50/50. The other KS user, Smeargle, never runs this move and is very uncommon in OU. KS isn't a broken move, KS is a broken move on Aegislash.
To see Aegislash back in OU would be cool and I'd like to, but banning the move King's Shield wouldn't make sense. I don't know if Aegislash deserves to comeback in OU because he seems borderline broken.
The question here is if King's Shield is really a broken move. Protean was obviously not broken since Froakie wasn't too powerful, but King's Shield issues are not the weak 50/50 arguments, it is the fact that a normally hopeless situation can be turned into your favor which shouldn't be possible.
If you are about to get swept by a Mega-Sharpedo, then your GHOST-type can stop the sweep. This is ridiculous.
Now imagine Gliscor would get King's Shield. I could half the attack of an Azumarill, toxic-stall and protect myself from damage. After that, I can wall a strong water-Pokemon with a ground-Pokemon. This is the aspect which is arguably broken on King's Shield and I think it deserves a test too.