At some point I think Smash has to shift its focus to something other than new characters, which will probably be for the inevitable Smash 6. It's going to be incredibly difficult as characters are by far the most hype part of Smash, but it just can't keep on adding ~15 new characters every time, it's simply not feasible in the long term, and not just because the roster becomes bloated, but also because good options are thinning out rapidly. Like, if they were to make cuts for that hypothetical Smash 6 it's probably not going to be worth it when the newcomers are all C or D tier stuff.
What it might shift its focus to I have no idea, but I do know that it can't keep up the same formular for much longer. I don't feel like cutting veterans is the solution either, I think Smash 4's DLC showed us quite clearly that cut veterans become huge favorites amongst the fanbase, so I don't feel like newcomers will be able to fill the gaps they leave behind, people will just want the cut characters over them instead.
I don't think it's a problem yet, so it's not something we need to have extensive discussion about now, but it'll be very interesting to see what speculation will look like for the next game.