nowhere did i say that aegislash needed to beat lando-i. i simply said that it helped combat 2 very big threats to the metagame. Sure, you could say that bulky chomp and av slowking beat gross, but those are two niche mons. Bulky chomp is slowly making its way here, but nobody is going to over-centralize so much as to put AV slowking on every team just to beat gross. Aegislash is a blanket check to these mons. Your theory about kang is definitely interesting... Sure, kang can't switch in on them, but can they switch in on kang? Frustration/EQ easily takes on these 2 mons. What mon would help Kang in this situation though, where it needed a fighting immunity? the sword and shield. I really dont need to not look at it from a non-linear perspective, because thats all it is: a straightforward discussion. Aegislash helps beat mons that were troubling us before and you can't deny that. If anything, Aegislash helps beat this constant onslaught of physical mons that so tarnish the metagame: Scizor, Terrakion, Tyranitar (can't switch in), Breloom, Pinsir. It also stops the draco spam that is the lati twins and aids in the removal of Gothitelle.
Slowking as a niche mon is debatable, but Garchomp is an A+ mon right now, and I'm pretty sure Tankchomp was his most used set and the big reason he jumped from A -> A+ recently.
Also, the problem with your Kang argument is that Aegislash certainly made it better, but it would be broken even without Aegislash's support, unlike, say, Lopunny. Aegislash made getting Kangaskhan in easier, but worst case, you'd sac and bring Kangaskhan in. As an offensive threat, it typically took a much greater cost to force Kangaskhan out than even our current S-Rank threats require.
And the problem is that Aegislash beats some problem mons from before, but he amplifies the effectiveness of the remaining ones that preparation is arguably harder. Let's just look at the S-Ranks (bar Clefable, who moved up just recently).
- Metagross: Needs to run Earthquake now, which while viable, is the lesser of the coverage options for that slot.
- Altaria: Usually ran anti-Steel coverage of some kind regardless of set, but Aegislash is fat enough to check it regardless
- Landorus-I: Eats opposing Aegislash, while Aegislash on his side counters and removes most of his other checks/counters like the Lati's, Slowbro, and Metagross (to name some)
- Keldeo: Choiced sets hurt a bit, but only in so far as watching coverage. They spam Scald/Hydro Pump the majority of the time, and SubCM loses little. Again, Slowbro/Slowking, the Latis, Unaware Clefable, Mega Gardevoir, etc.
So two S-Ranks (who come with slightly more opportunity cost as Megas) lose out on Aegislash (debatably in one case), while two others gain a teammate who eats their healthier-for-the-metagame checks and counters without being a problem for them to fight themselves. Maybe now I don't need 2 checks to Gross, Alt, Keldeo, and Lando per team, but now (from a smaller pool of viable mons), I need maybe 1 check for Metagross and Altaria (due to their lessened versatility), but 2 each for Lando and Keldeo, who had overlapping options (like Slowbro or the Latis) invalidated, making prep for them even more of a strain.
And this is ignoring other mons that might climb to S-Rank with Aegislash around (Bisharp and Lopunny immediately stick out).
The "match-up" issue is only being looked at in one aspect, that of
how many mons to prepare for. Aegislash reduces this aspect of strain, but he accentuates the other:
the amount of preparation each mon requires.
Aegislash creates a metagame with half as many threats (himself included) that each requires twice as much support. This seems like a fair trade, but with half as many viable mons, there's much less to fill the options for that "twice as much support" on a team, which makes team building extremely linear to be effective. And Aegislash himself is extremely versatile and splashable, but he can only feasibly provide so much of that support. Aegislash pushes most defensive mons down, but then you face overloading him since he's taking roles from at least half a dozen defensive mon. The result is the an increase for Hyper Offense, a playstyle already built around overloading entire teams, much less one prominent Pokemon, even as powerful as Aegislash.
And if Aegislash is indeed strong enough defensively and supportively to wall a number of threats that take 6 mons to do otherwise, I'd think that would fall under the broken definition for defense. We either get a broken defensive Pokemon, or an overloaded Blanket check that isn't even blanket checking the mons we consider the problem.