Much as I do love the sword, I don't think he has a place in an OU I'd like to play.
First, let me draw a comparison to the April Fools' Suspect. The reason people threw Giratina-O onto teams was because inherent attributes like its typing and uninvested bulk allowed it to basically blanket check 70% of the Meta, and depending on which of its highly flexible and different sets you run, it could check another 15-20%. Aegislash might not work with numbers quite that ridiculous, but he's in a similar boat. Aegislash almost invalidates half of the tier on his own merits (Fairies, Psychics, Fighting types and almost anything physical all hate him). Now consider what happens for offensive threats if they have a teammate that, without being specific or needing significant help of its own, can eliminate all their checks and counters for them?
Yes, Aegislash checks things like Mega Metagross and Gardevoir, but he also beats a good deal of their answers (Slowbro and Skarmory, for example). Metagross doesn't run Grass Knot as much because there's more he needs to handle than Hippowdon, but if Aegislash deals with them all for him, you'll just see Grass Knot become standard and partnering him with the Sword. So teams will heavily be based around a core of Metagross-Aegislash-(Answer to Aegislash), for one example. And this is just for examples that have a chance to contribute even with Aegislash there. Other mons like Serperior, Dragalge, Celebi and Starmie are just SoL.
And these are just threats that Aegislash checks but can partner with. I haven't even touched on the tier's current best: Landorus-I. He still partners well with Aegislash by dealing with his few checks/counters pretty well, but unlike the above mons, Landorus mangles Aegislash as well, reducing the effective core to Landorus-Aegislash. Landorus is already a Pokemon I debate needing a Suspect of his own, so I'm not a fan of reintroducing something that both directly and indirectly makes him better. I don't find the metagame so match-up dependent that I flat out auto-lose a bad match-up, whether or not it'll be an uphill fight. With Aegislash, there is no reason not to throw him on your team because there is no chance of you succeeding as consistently without him as you would with him.
Metagross the most recent suspect candidate, was elected to keep in OU, which tells me that there doesn't seem to be a belief that the Metagame is unhealthy enough to need a radical shift: healthy or not, no one's going to argue Aegislash would have a dramatic impact on OU. Every Metagame is inevitably going to have some degree of centralization, but the ORAS metagame has it in a manageable degree. Heck the arguments about match up mean that the centralization is probably less constraining than usual.
How much has changed that would truly make Aegislash less unhealthy for OU? Nothing that was introduced for ORAS' additions actually made him worse (he shits on just about every viable new Mega, forcing even Lopunny to play 50/50 w/ King's Shield), so there's no guarantee the game would be particularly different from his last presence. I am genuinely concerned that bringing Aegislash back could essentially put us back to the state the Metagame was in with him in early-mid XY, throwing out any balancing work the other suspects have been attempting to achieve. The only real difference would be the absence of Greninja (who some people considered an Aegis check, only another point in the sword's favor). As all the mechanics and pieces stand right now, Aegislash essentially locks the metagame down in its place, because unless a slew of additions hurt him, he's not hard pressed to adapt and keep the game wrapped around his hilt. The banner in the viability thread still reads "XY OU", and if we bring Aegislash back, it might as well stay that way, because we'd be right back to that Metagame with him around.
Keep the Sword in Ubers.
This argument is flawed as it can be applied to A LOT of mons. I will use your words as an example.
Mega Lopunny
Do you like umbreon? Fast offensive mons? Hydreigon? *Mega Lopunny* makes it unviable
Mega Alataria
Do you like umbreon? Do you like most non scarf mons named ferrothron? *Mega Altaria*
First off, Umbreon is unviable for a lot more reasons than Lopunny and Altaria.
Second of all, every Pokemon checks things yes, but Aegislash basically slaps down half of the Metagame just by existing. His presence in the tier, much less on teams, knocks the wind out of almost every Psychic type, and is a hit to Fairies and Fighting types. See Celebi and Jirachi, sitting in the A Ranks right now? Those two were either low C's or UNRANKED when Aegislash was around. Hell, I don't even remember where the Latis, consistent A/A+ threats (Latios was S for a time) were during the Aegi Meta. Lopunny is a check to Hydreigon, but he's still managing a decent B-Rank despite her. That's a significant gap in hinderance.
Certain Mons are hurt by things like Altaria and Lopunny, but their issues with those can be mitigated through team support because those threats don't have what it takes to break the Metagame on their knees consistently.
It should be noted that Aegislash centralizing the metagame by acting as a blanket check to a large number of threats is no different than pokemon such as Landorus-T and Clefable who do the exact same thing. So this argument should be discared whenever it's brought up.
This comparison is flawed because these two do not blanket check as much as well as Aegislash does. Landorus-T beats different things depending on his set, and still has to be played well since he's either a Scarfed pivot with only okay offensive presence, a defensive pivot without recovery, or a sweeper with some crippling weaknesses and a less than stellar speed to work around in setting up.
Clefable needs to set up in order to pose a threat, and while it can check mons by absorbing their hits, Clefable is also extremely easy to exploit with options like Heatran.
Aegislash blanket checks a huge number of threats, but he also can blanket counter a whole lot. I've already affirmed what types he shits on, and eating attacks off those Base 150 offenses or breaking 60/150/150 SubToxic is equally problematic. Unlike Clefable or Landorus-T, Aegislash does not need support to do his jobs, but supports teammates IN ADDITION to checking such threats. What the games eventually come down to is making sure you support the team against Aegislash checks, so that while Aegislash wears out your win condition's obstacles, you wear out the mons your opponent is forced to use to deal with him, as well as their own sword.
And this is all just on the basis of natural bulk and Typing. Aegislash is borderline uncounterable because each of his sets have different (and often shaky) answers: Chesnaught beats the old Crumbler set, loses to Flash Cannon; Hippowdon beats offensive sets, loses to SubToxic; Bisharp beats SubToxic, loses to 3-Attacks w/ Sacred Sword; Amoongus beats SubToxic and some Special sets, he's set up fodder for SD.
Aegislash isn't like Greninja where he's picking counters, Aegislash is picking his CHECKS, things that can stop him temporarily. I know being uncounterable isn't the only argument for being banned, but Aegislash can't be stopped from doing his job consistently, and he does very important jobs so well that stopping them is necessary to have a chance against his team.
