Aside: so, Ashe showed back up in the second post-time skip mission for me in Golden Deer, after disappearing as I mentioned earlier.
SpoilerI was able to re-recruit him after beating him, just like Lysithea on the Black Eagles route. Very odd though, I don't know why he had to leave for that. He didn't do anything special for the story or mission, just remarked about how it was his duty to fight in his adoptive father's place, then was strangely just fine with joining me after I offered. *shrug*
Oh, they will definitely stop that - doing "everyone is here" is not possible to keep up forever. I guarantee you the only reason they could do it with Ultimate is because they were able to re-use a lot of assets from Smash 4. There's just a practical limit to how many characters you can design and implement in a fighting game without the development time and costs getting to be too high, and Smash Ultimate is already well above the norm there. The next time they need to rebuild things from the ground up, we'll see a much smaller roster, it's simply inevitable.
And I'd say that Byleth is almost inevitable - the main character of a game always gets into Smash before anyone else. And as Razade pointed out it's really easy to make Byleth unique, since the Sword of the Creator basically means you can design his/her moveset like Ivy from Soul Calibur, which is quite different from anyone else in Smash. Plus their special class is proficient with brawling and faith magic too, so it wouldn't be out of line to draw on those fighting styles for moves either. I'd agree that Edelgard's popularity means she'd be first choice among the other three though.
I think Claude's certainly popular enough to get looked at - he's one of the only non-Black Eagle characters that I've seen break into those "most deployed" lists (it's usually the Black Eagles minus Hubert, then either him or Lysithea somewhere near the bottom of the list, in my experience), which is especially impressive since he's only available on one route, where most of the rest can be recruited on all of them. And if they get to that point, might as well go all-in with all four.
In the most extreme scenario I could even see them actually dropping everyone else from Fire Emblem for just the four Three Houses leads, though I don't think that's likely. It would require a new game with a much smaller roster, a stark limit to the number of Fire Emblem characters they're willing to put in, and the decision being made that going all-in on Three Houses was preferable to doing a list that's 2-3 returning characters with Byleth and/or Edelgard.