of course they would need to justificate the intial lack of content if they go in that direction, but we can look that the games are suffering in content and quality because the dev time that the roster needs is increasing each time, like we lost character unique modes, the story mode(both adventure mode, and subspace), more music is being copypasted, same with stages, and trophies, worse modes in terms of diversity(smash orders,smash target,smash tour,etc), even the characters that arent new are suffering,like in terms of alt,moveset,customs,etc; i dont know if that is justificable for only 10-15 new characters, even the balances suffers and we need to wait like 2 years before game is aceptable balance wise.
For the people that says that nintendo wouldn't do that, they already do that with splatoon 1, 2 and ARMS, hell even kirby is getting free DLC, i dont saying that the next game need to be barebones, or that they not add newcomers, but i think that this type of game would benefit the game in the long run, no, they not need to add lootboxes or microtransactions.