AAA Illusion vs Inheritance Illusion
The power level of AAA is
much lower. If something is carrying Illusion pretending to be some other Pokemon, you're not dealing with "Gengar is actually Banded Knock Off Tyranitar", because Tyranitar can't
get Knock Off. Nor is Gengar pretending to be Banded Hustle Extreme Speed Ursaring, because that's
not a thing in AAA. In general, in AAA mistakes are generally easier to recover from, because instead of the best moves being found on the most powerful Pokemon, you get... Standard, but with access to arbitrary Abilities, with pretty much any Ability that's 50% or more of a damage increase being either restricted to meh moves or banned outright, so really the power level isn't that much higher than in Standard. Certainly you don't have to fear random Quiver Dancers, random Tail Glowers, random Dragon Dancers...
As such, Illusion in AAA isn't really all that terrifying. If you assume it's an Illusion and it isn't... probably you're fine anyway, because the non-Illusions just aren't "you gave it one turn and now you're screwed". Nor is "Nasty Plot Illusion Gengar" even a thing in AAA -Gengar doesn't
get boosting. In fact, in Standard/AAA, here's all the Pokemon in the
entire game that have 120 Special Attack or more and Nasty Plot. (ie instant +2 to Special Attack)
Azelf, Beheeyem, Darkrai, Deoxys, Hoopa, Houndoom-Mega, Lucario-Mega, Porygon-Z, Thundurus
So, two Megas (One of which is banned), two Ubers aside from the Uber Mega, Hoopa, Thundurus, Azelf, Beeheeyem, and Porygon-Z. Beeheeyem is almost never used for anything and is hilariously slow, so it's not exactly going to 6-0 your team after one turn of setup. Porygon-Z is held back by its STAB being awful as an attacking type. Azelf is... painfully fragile, and still largely used for roles like suicide lead.
Thundurus's movepool has deep flaws, and in particular it has nothing for Magic Bounce Chansey if it's running Illusion. If I
generously assume it's Thundurus-Therian, we get...
+6 252 SpA Thundurus-T Thunderbolt vs. 4 HP / 252 SpD Eviolite Chansey: 324-382 (50.4 - 59.5%) -- guaranteed 2HKO
+6 252 SpA Thundurus-T Focus Blast vs. 4 HP / 252 SpD Eviolite Chansey: 576-678 (89.7 - 105.6%) -- 37.5% chance to OHKO
... so basically if you're stupid enough to let Illusion Thundurus-Therian get
three turns to setup for free, which Illusion can't reliably buy it, Chansey is no longer a useful check to it. That's
something, but Thundurus really likes a number of options a lot more for their utility against other things, and Thundurus-Therian isn't even that fast
anyway.
So, I guess maybe Hoopa-Confined running Illusion might be a terror in AAA? It
does have innate Psyshock, letting it beat Special walls. Even so, that would be a motive to ban Hoopa-Confined, not Illusion, in AAA.
Here, in Inheritance, anything can be an Illusion running Nasty Plot, and it can have any value of Special Attack all the way up to the highest legal Pokemon in the game. Gengar has 130 Special Attack, suddenly backed by Nasty Plot. You can't get that in AAA, full stop, and Adaptability is not
remotely equivalent. (Oh, and you can't be Illusion+Adaptability anyway)
Or... it can be
Swords Dance and Physical, again with monstrous Attack. That Gengar is actually Sucker Punch Swords Dance Tyranitar, and the one turn you messed up on has ensured it gets at least one KO before you force it out or kill it.
Context matters. Illusion
per se can be spread around just as readily in AAA as in Inheritance... but in Inheritance it brings with it a movepool that includes powerful boosting moves on
both ends of the spectrum, decent attacking moves (Again, on
both ends of the spectrum), and Trick. Not so in AAA. Even if it
did do all that in AAA, because magic, the punishment for thinking something is an Illusion when it's not is just... less. You will
never be surprised by a Tail Glow in AAA, unless you can't be bothered to keep track of what Pokemon can learn Tail Glow in Standard. In Inheritance you get powerful moves on powerful Pokemon with powerful boosting, all at once, with no guarantee of a warning beforehand. (Geomancy Power Herb at least has the good grace to announce itself with Fairy Aura. Tail Glow and Belly Drum provide no such warning) Illusion instantly makes all these powerful setup sweepers more threatening, by virtue of you being unable to assume you're not looking at an Illusion instead!
Thinking of it as "Illusion in AAA and Illusion in Inheritance" is completely missing that it's "Illusion in AAA, inheriting from Zoroark in Inheritance".
tl;dr version: Comparing inheriting from Zoroark in Inheritance to taking Illusion in AAA is a specious argument that completely ignores how radically different the choice being made is between the two metas
and the differences in the context of each meta.
They are not equivalent.
Everybody who is thinking the comparison works and we can use Illusion's relevance in AAA as a basis for making a decision in Inheritance is making a mistake. For that matter, an argument like "Protean was banned in AAA, we should ban it in Inheritance" is
also specious.