There is no way they will be able to keep every character forever. It's fine to dislike it, but it does get grating to constantly put it in people's faces. Don't worry about it. It sucks if you lose your character, but moaning about it doesn't help anyone. Just say your piece and move on.
Besides, you can always make a blog and rant if you got a lot to say. Important part is to remember is that it's just a game with limited resources and time. Ultimate was a special case to begin with. It's not a normal expectation for "everyone is here forever" in any fighting franchise. Smash was never different. The actual oddest time was only Brawl, and only because it was fairly early on in the franchise. Plus, you had issues with people thinking some replaced others(which only happened one time anyway, with Toon Link actually replacing Young Link, though as we know, they aren't just a graphical upgrade like Ganondorf, Zelda, Sheik, and Link were. Yes, they now resembled their GameCube counterparts, but still).
The cuts were also more of a unique case; most were coming back bar 2 at best till the development went awry. Meanwhile 4 didn't have that same factor, as some just were not possible thanks to the 3DS, and some were cut due to the state of their franchise otherwise(Wolf and Lucas, with only one returning as DLC). Smash Ultimate's actual tagline is "Everyone is Here". That means the tagline is specific to the game. Naturally it's not going to be a repeated tagline. Even if we get another game where somehow they can get all returned(including through DLC), it wouldn't use the same one. It's a very special case.
Basically? The same thing won't happen. This doesn't mean the next game "must have cuts" either. It means it's in a different position. Characters are very expensive to license, and it took multiple years to get that. Some of the 13 DLC characters could become base, sure. Some are in better positions otherwise(Hero no longer has as hard of licensing of music alone), and some being 1st party makes it easy. Whereas others are definitely more difficult(like Sephiroth and Sora), and other base game characters don't have the same situation(Cloud may have solely been in Ultimate base just because he was able to be negotiated for both games at the right time. But this does not exist anymore. On the other hand, if development started early enough, Sora could be in that position, but it's pretty unlikely he wouldn't be DLC anyway, since that's heavily how SE likes to use the character. That doesn't mean it's impossible, but there's no good evidence it's likely).