I simply think they have someone else in mind to break that barrier. I honestly don't think it was a coincidence that Metaknight, Pit, and Wario broke the trophy barrier, Charizard the Pokeball barrier, Little Mac the assist trophy barrier, Mewtwo the trophy barrier in the same game, and Inkling the Mii costume barrier. I think who they are choosing is very deliberate and, with this in mind, I think Waluigi runs away with it. Him being an AT is practically folklore in Smash culture at this point. It's almost canon that he WANTS to be playable but keeps falling short for one reason or another. The fanbase has happily filled in the gaps, especially for Ultimate. He gave his invitation to Daisy, Plant/Joker stole it, etc. None of the other ATs really have this mythos around them and even Nintendo itself has commented on it. Waluigi in Smash is more than just a character request, it's a ****in' saga of Nintendo's perpetual loser that just wants things to go his way one time and that's compelling in its own right. Waluigi isn't "real" but the fanbase has created this whole lore about him trying and failing to get into Smash, only to keep trying.