Mii Costume theory, despite me not liking theories, isn't dead.
The needle simply didn't really move one way or another. We still only got Square Enix costumes, and the old Square Enix costumes have a logical explanation as to why they aren't there.
Joker, on the other hand, brought us older costumes. Not all them, as they also received cuts. I don't think i'd discard Mii Costume theory because the stingiest Smash collaborator didn't provide their old stuff.
I don't think the theory itself necessarily gained credibility. However, to call it "dead" is getting ahead of ourselves if you ask me. We still have no word on:
Monster Hunter costumes
Mega Man X
Proto Man
Zero
Heihachi
Lloyd Irving
No offense to anybody, but I think this is a case where folks are looking for a reason to kill it so that their most wanted seems more likely. That's your prerogative. But there is literally no reason to act as if it's entirely dead.
What would ACTUALLY KILL IT, is if Banjo gets revealed with Capcom and Namco costumes. I would wager it isn't going to happen though, and that Banjo will be getting Banjo-Kazooie costumes, Minecraft, and MAYBE...MAYBE Halo. I doubt that last one though.
I will say it again: Mii Costume Theory is entirely dead if costumes from an unrelated company are revealed alongside a character from a different company. Example) Banjo is revealed, Mega Man/Monster Hunter follows. (Before you say, "Atlus isn't SEGA!", Atlus is owned by SEGA, pretty much the same thing)
There's still Capcom and Namco content on the table. Until those are shown without a fighter from those companies, Mii Costume Theory will live to see another day. Not trying to dampen expectations, but let's at least assess it accurately before we say it's "dead". We still have only gotten Mii Fighter costumes from the same company as the revealed fighter, and there are old costumes left on the table.
Old costumes don't have to return, as evidenced by Virtua Fighter and now Chocobo and Geno. But they may, as evidenced by Tails and Knuckles with Joker.