I think banning Aegi takes a step towards a more defensive metagame? It's much more difficult to deal with Aegi in its entirety on stall than it is on an offensive team where at the least you can fit a couple checks and not have to have perfect swaps.
Your assessment of Aegi's best sets is subjective. I've heard people who think that LO is the best Aegislash set (like Toxzn). Your assessment is based on how you build and what you like to use, because there's no way ANYTHING is objectively Aegislash's best set. Lando-T, godly Aegislash check that is outsped and 2HKOed by LO Shadow Ball. Excadrill, stellar Aegi check that can at best revenge kill because it isn't switching in ever, and fears Air Balloon. Mandibuzz, stellar Aegi answer that loses to three movesets (and even the KS attacks one if that has Toxic). Bisharp is a fine Aegi check I guess, though is outsped and KOed by fast sets. Zapdos, which is exploitable af because you can just waste Roost PP from the 50% you do and eventually get rid of it, or pop with a Toxic. Torn-T which can't even do shit so idk what you're saying ?_? Greninja which has to forgo coverage for Dark Pulse if it even wants to hit it that hard. Lando and Chomp are fine. All these amazing Aegi answers! There's no way in hell you build a team and are like, oh, I have Aegislash all covered. It puts constraints on your teambuilding to carry multiple of these checks and is one of the primary things you have to actively think about. The other thing is that Aegi weak teams are usually bad, more so than Thundurus or something which are easier to check and deal with, and don't have such different sets that you need answers for all of them. I am not forced to carry 3 Thundurus checks on my offensive team, but I almost have to (or at least 2) if I want not to be Aegi weak. Because Aegi is on a whole other level than those Pokemon. Anyways I don't think this metagame tree is healthy at all.
Aegislash
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Things that beat Aegi or at least do decently against it
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Things that lose to Aegi
Because this is exactly the polarizing effect Aegislash has on the tier. Liiiiike BW wasn't
Drizzle
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Things that beat Drizzle
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Things that lose to Drizzle
Having multiple centralizing forces in a tier (say a group of 4-7 Pokemon) is not unhealthy, but a tier centering around one Pokemon is unhealthy and illogical (before you bring up GSC Snorlax would be banned by today's standards). Because the metagame becomes stale (let's be real, how much has the meta changed recently? same boring stall builds, similar offensive builds) as teams are generally better when they run similar builds that are prepared for this one threat and the metagame around this one threat.
Basically you are underplaying its versatility and the overall staleness of the meta that is so much of Aegi's fault.
I don't agree that stall has a hard time against Aegislash, not when they typically carry 2-3 checks or counters to it. I am talking about the Lefties sets btw, not the LO set, because the LO set is a dedicated wallbreaker that fares a lot worse against offense, and gives up the ability to check any Pokemon you could before with a few exceptions. Basically, Aegislash will either have to choose between being more effective against offensive teams (with Lefties sets, or even Air Balloon), or being more effective against defensive teams, but it can't do both at the same time. And if we are talking about dedicated wallbreakers that can also be somewhat useful against offense, we already have a ton of them, such as Kyurem-B, Char Y (needs Pursuit support but fine), SD Mega Mawile, Knock Off and Calm Mind Landorus, Diggersby, and Mega Gardevoir, as well as a lot of stallbreakers. Aegislash is nothing special in the wallbreaking department, and thus doesn't really trouble stall teams anymore than any good OU wallbreaker that stall teams need to prepare for.
On the other hand, without Aegislash, a ton of solely offensive threats become way more effective. We are talking about Latios, Latias, Mega Pinsir (now able to use CC to do more damage to Skarmory and Rotom-W, two if its biggest counters), CB Talonflame (one less bulky Flying-type resist to worry about), Mega Scizor (more flexible moveset, as Knock Off is not a must anymore),Terrakion, Breloom, Mega Gardevoir, Mega Heracross, Mega Medicham, Lucario, LO Tornadus-T, and Staraptor. Most of those Pokemon lose one of their biggest counters if Aegislash leaves, which greatly improves most of them. I hope i don't even have ton talk about how better Latios, Latias, Terrakion, Breloom, Mega Gardevoir, Mega Heracross, and Mega Medicham will be with Aegislash gone.
And what defensive Pokemon become more viable to compensate for this? Not a lot really, as most Pokemon weak to Ghost are not good defensive Pokemon anyway, due to their unfortunate weakness to Pursuit and Knock Off, and some other factors (Celebi facing big competition with other Grass-types). And even though some defensive Pokemon that were Aegislash bait (Chansey) won't have to worry about this anymore, the fact that they will have to worry about a dozen improved offensive Pokemon is what matters more, and what leaves an even more offensive metagame in the end. It's not like there aren't good Aegislash checks or counters to form a solid defensive core with, but good luck like trying to deal with Pokemon such as Mega Gardevoir, Mega Heracross, Terrakion, Mega Medicham, and SD Lucario with defensive cores, or cores that rely on defensive synergy in general.
Regardless if the assessment i made is subjective or not, the fact that great and multiple universal Aegislash checks exist can't be denied. SpD Gliscor, SpD Hippo, SpD Amoonguss, and SpD Mega Venusaur can deal with any of the three of Aegislash's most effective sets (3 attacks, SubToxic, mixed), while all of Aegislash's sets are easily checkable with two Pokemon, something you need for every major threat. There are a lot of universal Aegislash checks, and a ton of checks to it in general, to blame Aegislash for punishing wrong switches too hard with its unpredictability.
Also, i like how instead of focusing on the universal checks i mentioned, you focused on the situational checks, and of course you used Aegislash sets that beat those checks to show how shitty checks they are, as if they are supposed to take on Aegislash by themselves. Oh and AV Torn-T has Knock Off and Heat Wave to deal with 3 attacks Aegislash, while being able to easily tank two of Aegislash's hits, and in general it can scout any Aegislash set with minimal risk, except from the SD set, so yeah it's a very useful Pokemon to use in combination with another check to deal with Aegi.
It puts constraints on your teambuilding to carry multiple of these checks and is one of the primary things you have to actively think about.
Every major threat does that, this proves nothing about Aegislash's brokeness.
As for Aegislash weak teams, such teams don't exist, because they are shitty in general, not only against Aegislash. If you are talking about teams that are only a bit weak to Aegislash then this is manageable and can be dealt with by good play, and is also true for any top tier threat. All teams are weak to something good, but as long as you are only a bit weak to it and have ways to play around it, this threat doesn't reduce said team's effectiveness. And the reason that teams that are really Aegi weak don't exist is because there are a ton of top-tier threats that check Aegislash, threats that do a ton of stuff other than dealing with Aegislash, and threats that you put on your team without even thinking about Aegislash. This is what happens when you are weak to three of the best attacking-types in OU and you are slow as fuck.
Finally, what you said about Aegislash is an exaggeration, because you only need 1-2 Pokemon to deal with it, and then you have 4 slots remaining, which can be anything like: 3 neutral Aegi mons + 1 weak, 2 neutral + 2 weak, 4 neutral, or even 3 weak and another check. But your last statement is very generic and subjective and can't really be argued, so i will not expand more.
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So basically, what i said in a nutshell is that Aegislash has a lot of checks and counters that prevent it from being an overpowered Pokemon in OU (which means that it's not too strong as a Pokemon separately, whether this means offense, walling, or support), and also makes the metagame less offensive that it would be without it, which makes it a positive presence in OU (this is subjective, but i think that the metagame doesn't need to get more offensive).
