Controversial thing to say, but people really oversell how having an Assist role helps for the future.
If anything, it's the opposite. The ratio of Assist to playable fighter is downright abysmal if you actually do the math.
Out of 48 Assists that have been in Brawl or For, only
3 have made the jump. And the ratio is only going to get worse when we factor in Ultimate's new ones once the next game is out.
And the three that have happened were extraneous cases.
: Punch-Out!! for the Wii happened between Brawl and when For's design document was drafted, reviving the series (albeit temporarily; I guarantee Mac wouldn't be in now if he didn't get in For).
: Literally didn't exist when For's design document was drafted, so she COULDN'T have been picked for For as a fighter to begin with. But since the design document, New Leaf had been released and Isabelle became one of the faces of Animal Crossing if not
the face.
: Only got in because Echoes were a thing for Ultimate and happened to have demand from the Ballot to warrant the idea.
I'm not including Charizard in this because if I did, the ratio would be much worse factoring in all the Pokémon that didn't become fighters (as Charizard is literally the only one and it was as part of
's gimmick).