After laddering for quite awhile and almost at the reqs 2371 lmao don't judge and seeing the aegislash metagame myself, I'm going to add in my two cents to the discussion of whether or not aegi is meta defining or deserves to be banned. I hope my introduction and first time post count isn't discouraging enough to find my opinion biased or the like, however this is my opinion, which I will be mainly leaning towards as an anti-ban perspective, and indeed I have lurked offline during the first aegi meta.
One issue I have with aegislash's presence in the metagame is that it is very overwhelming and difficult for teams to prepare for. Aegislash, unfortunately, has no counters, and turns reliable checks into liabilities with it's 50/50 chance of utilizing King's Shield. However, I cannot say I want him to be banned for these, undeniable truths. The issue in today's metagame is that threats are more versatile than anything we've ever experienced, any one pokemon can successful run a lure set or bypass it's commonly used counters with some kind of move, whether or not it is a viable choice for a moveslot in common practice.
One example I use of this, is heatran. Heatran has amazing typing, fire/steel gives it immunity to poison, and giving it resistances to several common typings that plague the metagame, particularly dragon, fairy, steel, flying, and grass. Heatran is highly versatile and meta defining, one can say it is uncounterable, literally, scarftran can catch anything off guard, air balloon tran can cripple garchomp leads with HP ice or status, tran can run a stalling set, a supporting pivot with stealth rock and being a reliable switch in while spreading status, and is also a blanket check to a large majority of the tier such as serperior, talonflame, volcarona, and pretty much every fairy not named diancie (which if it's scarftran, can flash cannon before diancie earth powers). Heatran offers plenty of utility in the metagame with minimal crippling flaws. However, in no way is heatran broken, or considered overcentralizing despite making pokemon such as volcarona completely useless when the pokemon is present in the team preview. Serp can run HP ground, but that's specifically and only for heatran, which opens up issues with other mons, zard X is forced to run EQ or else heatran counters, and natural gift talonflame is, yeah you get the point.
Personally, I think heatran's description matches aegislash, as heatran's only down flaw, is lack of being a mixed set, however I think that can be bypassed with the amount of power heatran's moves actually have and the crippling effect of them. Likewise to how heatran is a blanket check to serperior, volcarona, talonflame, venasaur, celebi, ferrothorn, skarmory, and if scarf; diancie, gardevoir, latios, and latias. Aegislash is a blanket check to several threats such as metagross, hawlucha, celebi, starmie, altaria, gardevoir, and jirachi. Just like latios has to give up HP fighting for bisharp/tyranitar for a coverage move to lure heatran, metagross has to give up hammer arm for EQ to hit aegislash on switch in. Altaria specifically needs EQ to hit heatran on switch or sweep, altaria needs fire blast to hit aegi's switch in hard (but it's good for ferro, skarm, and scizor too I guess). Heatran doesn't like focus blast from gardevoir so it can hardly switch in.. likewise aegislash equally doesn't like shadow balls. (What does garde run FB for other than heatran anyway? Aren't the pink blobs still toxic stallers?) Heatran doesn't want to eat earth power from diancie or celebi, neither does aegi. Latias is kinda screwed by both no matter what, but latias is screwed even by pawniard lmao, not everything that's checked/counter is going to reliably handle it's checks/counter easily. My point here is, with heatran's presence in the tier, we don't see a massive decline in viability here, other than maybe the necessity for more ground types. Aegislash, IME, has a similar placement in this metagame as heatran. Remove heatran, aegislash takes it's place as the tier's blanket check and pokemon can run aegi coverage over heatran coverage. Remove aegi, and pokemon don't have to run aegi specific coverage (shadow ball garde) instead of heatran specific coverage (focus blast garde). Combine the two, and a core of blanket checks is on your team and preparing for them becomes a nightmare. However I do not believe either pokemon is broken or meta defining enough, and heatran's long term presence in OU has proven that blanket checking the tier isn't exactly a good reasoning for aegi's ban alone. The synergy of two blanket checks in the tier may be a good reasoning, but you can't point fingers at this mon over that mon for the same cause and effect. I don't see volcarona or ferrothorn becoming unusable because heatran's presence is in the tier, and it has a fairly high usage statistic too.. saying hawlucha will become unusable (and others) just with aegislash existing seems over dramatic. Is air balloon heatran a metagame threat because it can come in on predicted EQs and hit the switch in hard? No, it's easier said than done as heatran is coming in to sponge hits and will most likely take chip damage to pop it, likewise for air balloon aegislash who still has to worry about dragon tail garchomp and u-turn/knock off from landorus-T. They both pass their one "check" hippowdon significantly better, but not enough to justify them unkillable. Even keldeo can't switch into heatran without taking a status move like toxic, similarly to how mandibuzz can't switch into aegi in fear of toxic and flash cannon. Moving on from comparing aegi to current metagame pokemon.~
50/50s are another argument I see from the pro-ban side, that is undeniably true. However, I find the 50/50 argument a little bit over exaggerating. I could've agreed with this before I laddered when I originally read some of the pro-ban arguments and prior to laddering.. but after laddering for sometime, I honestly had more 50/50 coinflips and yolos with predicting switches, double switching, Zard X or Y, and whether or not pursuiting or sucker punching was my best option with bisharp. Bring in my latias on the charizard thinking it's a Y due to lack of special attackers on their team, nope, it's an X that dragon danced and now the team is half dead before taking it out. Aegislash was the least of my problems because the event of it being a literal 50/50 wasn't that big of a deal since my knock off users either didn't care for KS (bisharp) or I was using something that didn't have a contact move but spammed EQ. One really has to stretch for a scenario that something is using a contact move to begin with in an EQ flooded metagame. Lopunny does get a little annoyed by KS, but even then there is work around sets with sub+encore, which are equally as annoying to predict when you're switching into something to sponge a predicted fake out or the like.
The fear of metagame threats becoming stronger due to aegislash isn't a good excuse to keep something out of OU either. We did ban Deoxys due to making terrifying sweepers like bisharp broken. However we narrowed down the issue wasn't bisharp, the one ripping the teams, it was deoxys's flawless hazard and utility support. With aegislash, I feel it's vice versa. I'm not really seeing the synergy with lopunny becoming a potent threat due to aegislash's influence on the team. Like honestly, neither pokemon have the right defensive synergy.. aegislash can take flying, psychic, fighting, and fairy hits, but we forgot these same pokemon utilizing these moves still can predict and hit with coverage. Before anybody says, "careful with the prediction argument", it goes both ways. The aegislash player has to be very cautious of his aegislash switch in too, there is a best and worst case scenario here, which equally negates itself out of the equation, we can only assume the worst case scenario, say, aegi doesn't come in when you use coverage. However, if we used worse case scenario as an argument, we could say parahax or confusion hax from tornadus-T and the like would be ban worthy solely on the "worst case scenario" argument for the opponent.. that they can pass their checks and counters while synergizing well with threats because the worse case scenario took place. Landorus-i does get better because of aegislash admittingly, however mamoswine is still a massive thorn to the core's side and landorus-i was always a potentially broken pokemon from the beginning. If an already broken Pokemon A makes Pokemon B broken because pokemon B supports pokemon A even better making it more broken, then it's not necessarily pokemon B that's broken, but pokemon A. Say aegislash is broken because it did it's job in removing a sweeper's threat, but we never banned gothitelle for removing pink blobs for terrifying special attackers to plow through either.
In conclusion, I really don't think aegislash is unhealthy, or broken at any chance. Overcentralization is easier said while looking at the viability rankings thread, than experienced and shown in practice. Rarely is aegislash the game wincon when there are still other options for this and that check available in the team slot besides him. However, I do agree aegislash makes team matchup still an issue with "just being another threat to prepare for," but if any pokemon we picked out of viability thread was removed from OU, then retested, I'm sure the same argument would apply as "just another threat to prepare for" and therefore I find aegislash as a contributor to not be an influential pokemon to team matchup, but equally every pokemon with aegislash being the lesser evil, and just another threat. I personally will be voting unban under the assumption and perspective that aegislash is more broken on paper than in practice, and I do not find him influencing enough to justify a ban over many other pokemon in OU with similar roles and capabilities. However, I will not deny that everything the pro-ban side said is perfectly logical and fine, just not accurate from what I've seen on the ladder myself, or from my point of view from the pokemon itself, as an aegislash user, and opponent.