I want to bring up one aspect of Aegislash that's entirely separate from brokenness, but paramount to why it should be banned imo.
50/50s
Pokemon is a game of chance, but XY OU is literally a tier of 50/50s, and it's so much because of Aegislash. King's Shield and the risk associated with staying in Sword stance or reverting to Shield stance exacerbate the amount of 50/50s on another level. Let's take Sub Kyurem-B. It comes in, goes for Sub on the attack and its Sub is broken. It can Sub on the KS and gain an advantage or EP on the attack and kill Aegi. Or Garchomp will SR vs Aegi and lose a crapload of its health for no cost or attack and bring it down to low health. Will Aegi Sacred Sword the Bisharp switch-in? Does Mega Mawile set up and lose a crapload of its health or does it attack and kill? Merely the presence of Aegislash forces 50/50s. Does Terrakion EQ the Aegislash switch or does it Stone Edge because Thundurus or Latios will come in? Not all of these are the best examples I'm sure but you get the point. I think a Pokemon that creates so many 50/50s tends to remove skill from the metagame and make it, for lack of a better word, "less fun".
50/50s are NOT skill-based. You're making a disconnect between good plays and a flat-out 50/50 where all you can do is guess. It's not even an educated guess, it is complete and utter guesswork.
If you're arguing against a ban due to collateral then your reasoning is fundamentally flawed. Keeping broken stuff to "tame" the meta doesn't really make sense. If it requires more bans afterwards, who honestly cares? We suspected for all of BW, and not once was a ban considered for collateral damage. Banning Genesect flipped the tier upside down in BW. This is irrelevant; the only factors relevant in a ban are the brokenness, unhealthyness, and/or uncompetitiveness of the testing subject.
People are downplaying Aegislash's versatility enormously. Please don't bring usage stats into this first of all. Aegislash has four good sets basically. The standard tank is one, where literally the only things guaranteed are Shadow Ball and King's Shield. Iron Head, Toxic, Shadow Sneak, Sacred Sword, Flash Cannon...even within variations on its standard set that DO NOT reduce its effectiveness against the majority of threats (think running Psychic > Focus Blast or HP Flying > HP Ice on Thundurus, or Sludge Wave/Rock Slide Lando) it can wreck its different counters. SubToxic turns the tables on Mandibuzz and is an enormously effective staller. Fast LO 4 attacks annihilates Mandibuzz as well, among things like Heatran non-full SDef Hippowdon Lando-T Chesnaught Bisharp etc. SD wrecks Amoon Mandi Heatran and is a dangerous as hell sweeper/breaker. All of these sets are extremely deadly and ridiculous to deal with. This is like Deo-S, except amplified five times. The fact is that when you see Aegislash, you have almost NO IDEA exactly what it's going to do is so different from anything else. I can see, say, Thundurus and immediately rule out a lot of things based on team comp alone. I can't do this with Aegislash; this makes its versatility even MORE of a concern, because it is so good that on many teams it can literally be running any of its sets. I know Deo-D is banned but let's just use this example. I look at a Deo-D team and Aegislash could be anything. Air Balloon? Maybe. What's the Air Balloon set running? Is it slower or is it faster? What moves is it running? Like what if it's SD instead of mixed? Or it ends up being Life Orb. Or the person wanted the defensive backbone/longevity for whatever and decided to run Lefties. So don't downplay Aegislash's versatility, it's absolutely an enormous factor.
On overcentralization: please name me one other Pokemon in the tier that is so prevalent that it forces every single Pokemon in the tier to account for it. Dragonite, Terrakion, Mega Pinsir, etc are all running EQ for one target - Aegislash. Damn Mega Gard is getting screwed by Aegislash, how worth it is it? To go to an extreme let's take Hawlucha. Obviously it has issues but you look at the tier and the first thing that pops to mind is Aegislash. How does this beat Aegislash? I guess it's not that good. Not ONE Pokemon in the tier other than Aegislash has this degree of influence. Diversity is not inherently good but Aegislash, as a Pokemon, places implicit limitations upon what's good and what isn't in OU. On the flip side, why is Bisharp so good? Obviously it has all these positive traits, but it is literally the only Pokemon able to Pursuit trap Aegislash -- albeit on a 50/50 and not even completely killing -- making it a million times better. Why is Dark Pulse Greninja even a thing? Just Aegislash, now that Deo-D is gone. I know this isn't really relevant to a ban at all, but thinking about what we'd have to do with OU Analyses after an Aegi ban...I can't even imagine how much would have to be changed. Aegi's impact and pull on the tier is just so large that XY OU is entirely different because of it. I can ban Mawile or Charizard or whatever, but none of these things will have that much of an impact. Even while the Deos which we just banned in the tier, they weren't ruling the roost - Aegislash was. Nothing compares to the overcentralization in OU around Aegislash.
There's a good reason most good players I have talked to are in favor of an Aegislash ban. It's literally the perfect Pokemon that limits teambuilding as it is so unpredictable and good at everything to the point where Aegislash IS the OU tier. This is unhealthy and makes it fully deserving of a ban.