I don't expect the next Smash Bros. to bring back every 3rd Party back. Some of them, sure. All of them. Definitely not. Especially in the cases of Square(due to the vast amount of licenses despite their willingness) and Konami(due to being a wild card).
That's not to say that the next Smash Bros. won't be good or will not have a pretty good roster on it's own. Like not just any new 3rd Parties that enter the roster. I mean, when you look at just the Nintendo side of the roster, it's pretty dang good. Like besides having the currant roster and return of many veterans like Pokemon Trainer and Wolf, and the Zelda cast being diversified from different games rather than Link, Zelda and Ganondorf being from the same game, we got frickin' Ridley, King K. Rool, Daisy, Dark Samus, Splatoon, ARMS, Pyra & Mythra, a Fire Emblem character that uses other physical weapons besides just a sword with Byleth, a wrestler character with Incineroar, etc.
Like being a treasure trove of 3rd Parties crossovers isn't just the only strength that Super Smash Bros. has. It still has a fantastic list of Nintendo characters, both in the playable list(with 23 series) and those yet to become playable. If they bring a bunch of wanted 1st Party characters that people have wanted alongside new additions from newer games, I think the reception will still be pretty positive even with some of the 3rd Party removals.
Kinda like how the Nintendo Direct was received this month. We got Metroid Dread, we got Advance Wars, we got WarioWare, we got a classic Mario Party game, we got Super Monkey Ball, we got Fatal Frame, etc. And alongside games we knew were coming like Breath of the Wild 2, Shin Megami Tensei 5, Monster Hunter Stories 2, Mario + Rabbids 2, etc. Like we got a diverse amount of games and game series shown off and the reception was very positive.
I think the next Smash Bros. can still be very good if Nintendo can back-up any 3rd Party removals with a diverse new lineup of 1st Parties and new 3rd Parties. We may not get Metal Gear or Dragon Quest, but we can still get other 3rd Party series like Monster Hunter or even Indie series like Undertale.
And after Little Mac, Dark Samus and Isabelle, who's going to be the next Assist Trophy to become playable? Is it going to be Isaac? Is it going to be Takamaru? Ashley? Krystal? Is Waluigi going to escape the Assist Trophy curse? Or if some 3rd Party series returns? Maybe Alucard? Maybe Zero? Maybe Shovel Knight?
And what is going to be the new Nintendo series to get a playable character? Golden Sun? Advance Wars? Astral Chain? Ring Fit Adventure? Like, sure we are all thinking, "Which characters are we going to lose?" first and foremost, but some of these other questions are also interesting to think about.
Like Ultimate is a legendary game all on it's own with it's vast amount of crossovers and whatnot. And it's going to be hard to top it off. But I think Nintendo can still make a very good Smash Bros. game after Ultimate if it brings just as many new series and unique characters. Heck, not just with potential new characters relating to series getting new games coming out like Advance Wars, but we still have the Smash Ballot. And the Smash Ballot wasn't just able to give us some very wanted character like Ridley, King K. Rool and Castlevania, but also bring some wanted characters as Assist Trophies that could potentially become new characters in the next game, like Isaac or Alucard.