I like how some of you are still fooled into believing that Aegis severely limits a select group of Pokemon. Starmie nor Tentacruel really don't gaf about spamming Scald rather than spinning (in Starmie's case, it's also perfectly fine with spamming LO Hydros all day and then spinning, like it's done for three generations already). Voir, Gallade, Celebi, and Rachi can still perform their jobs very well (Celebi in particular can just BP / dry BP to something that just straight up bops Aegis); and idk if people are lumping Slowbro in with the Psychics that Aegis makes "unviable," but it's definitely not being hindered much by Aegis. This was the case in XY, but the lot of you were too stubborn to realize it. Aegis does not dominate OU as much as you people make it seem, it just makes the tier a little harder. Honestly, I'd rather take that OU than spend years testin and bannin shit (and we don't even know if X2/Y2/Z whatever is gonna poop out even more broken threats / mechanics!).
Aegislash was the main reason why Jirachi was considered
unviable in OU, think about that. Celebi was also hampered by Aegislash, even with its access to Earth Power; Aegi's access to Magnet Rise only made it worse. Mega Gardevoir was walled by Aegislash unless it ran Shadow Ball, which rendered it vulnerable to many other Pokémon to a much greater extent (other Steel-types, mainly). Mega Gallade
could get around Aegislash with a well-predicted Knock Off, but since Aegislash can just King's Shield in Gallade's face, take jack from Knock Off and proceed to OHKO with Shadow Ball, that's yet another matchup greatly in Aegislash's favor. As for Slowbro: sure, it can inflict a Scald burn, but it's NOT going to enjoy eating a Shadow Ball at all. As stated, long-time OU veteran Starmie dropped to UU
because of Aegislash rendering it barely viable. It couldn't spin and couldn't inflict enough damage on it most of the time, while being
another Pokémon to fall to Shadow Ball. Don't even
begin about the countless other Pokémon that were rendered much less viable, if not unviable because of this bastard sword: Mega Medicham (quite big when Mega Gallade wasn't around yet), Mega Heracross, Terrakion, Hawlucha, Staraptor... Probably a whole ton more and ORAS only brought new toys for Aegislash to blanket check, including Mega Metagross (especially with Magnet Rise)...
If you imply people are stubborn for defending their perspective, then you are blind to the massive impact Aegislash had on the OU metagame; it rendered the entire metagame stale and static,
impaired the metagame's growth and development.
Gardevoir was by no means unviable when Aegislash was around, it could 2 hit KO with shadow ball on the switch. People just used it less because they couldn't mindlessly spam hyper voice for an easy win. Starmie saw low useage for many other reasons, and as someone that used it himself it could beat Aegi unless it ran shadow sneak. Sure you had to use analytic but it was by no means a bad set.
There were plenty of broken mons in the meta back then that severely impacted useage outside of Aegislash (not saying that Aegi is broken).
Yes, Gardevoir had to run Shadow Ball, but this meant sacrificing the highly important Focus Blast and either sacrificing stallbreaking utility (Taunt, Will-O-Wisp), coverage (Psyshock/Shadow Ball/HP Ground) or boosting (Calm Mind). Aegislash's
presence forces Mega Gardevoir into 4MSS, which just shows how it impaired Gardevoir's viability: no Shadow Ball meant no hitting Aegislash and losing much of its otherwise amazing potential. I could say the same about every Pokémon that had to adapt to Aegislash just because of its mere presence and thus, inhibiting their viability to a decent degree.
Even then, Aegislash easily exploited some of those Pokémon, so
how is that a healthy Pokémon for a metagame?
Moot point because several pokemon, namely medicham, heracross, pinsir, and a few others actually have lower viability (and tier usage) than they did during the x/y aegislash metagame.
Even mega pinsir who's RU by usage but BL for brokeness, was S-rank OU during the first aegislash metagame. With this pattern, the "Oh no shit is now unviable!" when it already struggles in a nonaegislash metagame is really biased.
Starmie is just more viable now cause it can effectively RK the latis, and somewhat deal with the massive speed creeping oras introduced. During the aegi meta, mawile was around, bisharp was more common, and the ladder was filled with cancer pass teams.
All of the Pokémon listed in your first paragraph are Mega Evolutions, so your comparison with the ORAS metagame, a metagame with a much greater range of viable Mega Evolutions, is inherently flawed. Mega Pinsir just suffered from getting far more viable competition for the Mega slot and, since it can semi-reliably deal with Aegi, might even rise in viability again if the goddamn sword were to stay.