I suppose the most comperable example would be MvC. UMvC3's roster is much smaller than MvC2's, and people still enjoy that game (IIRC the problems people had with it were due to on disc DLC and the fact that they didn't even wait 6 months before putting out the enhanced version, so I don't think they'd apply to SSB6).
Granted, MvCI's roster was even smaller and people hated that, but unless they cut the entire Metroid, F-Zero, EarthBound, and Star Fox cast for no reason (EDIT: and the game's visual style is fundamentally flawed) I think we'll be fine there.
Don't forget the main complaint about 3's, which is the function crap. That's really the worst part, though it's a great game otherwise. Mainly that removing MegaMan due to "being a shotoclone", which would've made sense, if they didn't quickly throw Akuma in(albeit, he doesn't miss a game, but a hypocritical point). They did thankfully give us Zero and a return of Tron Bonne, but it's still worth mentioning. It's part of why people accepted the game. They at least gave something cool in return.
That said, 2's was the last time the roster wasn't given any major complaints, but it didn't have really boneheaded decisions. At worst, too many clones. But even then, it was full of diverse and fun characters anyway. MVC One of course had the "where's Iron Man" bit, but it wasn't as bad as others. Mostly cause War Machine was still fun to use and we got a neat variant of him. That, and in some cases, War Machine is still one of Iron Man's Suits. So it's not like it can't be Stark in it.
Anyway, Infinite also had good gameplay, but that only barely saved it. That won't work with Smash, which has an even bigger focus on characters than MvC in general does(which is also part of its appeal, but it has tons of franchises moreso, and a rotating roster anyway, something Smash doesn't have. It's why the rosters are accepted easier. The main issue with Infinite was boneheaded cuts. You can't actually justify Wolverine being gone. It's fine if most of the X-Men aren't there, but he wasn't even DLC. People were quite justified in being pissed off).