A lot of people are shutting down first party talk and laughing at those who still support first party picks.
Well, there are still quite many 1st party characters and franchises for choice, Waluigi is just one of them.
Put the assist deconfirmation and 1st party deconfirmation theories aside, we still have another obstacle against Waluigi, that he isn't from a new universe, but from a one that has overpresentation in Ultimate. I wouldn't say laughing, but it can be a reason for other fanbases to not welcome his inclusion right now.
Personally, I would like to see Waluigi to fight more because of
my interest in him as an individual. Not that he must be paired with Luigi or Wario, Seriously, I think that idea should be executed in a Luigi Mansion or a Warioware title first if we're really upset that NIntendo don't take their relationships seriously.
Using pairing or relationship as sole reason is defective, since this can be applied to all other fighers who joined without their partners (Shulk no Riki, Yoshi no Birdo etc). We know, the slots are not enough for all partner/rival characters to join in the end.