I think this viewpoint is overly colored by the effects of a (very) long DLC cycle in which the focus on third parties was weighted much more heavily, in large part due to the ability to milk a third party character's profitability much more greatly by selling them at a lower content-to-dollar ratio. Even in base Ult we only got one unique 3P newcomer; the rules were notably different in DLC.
Now, clearly the floodgates have opened, and people are not so shy to suggest any number of left-field 3P crossovers nowadays. But I think a lot of the demand for 3P reps has been fueled by expectations rather than a legitimate lack of good 1P options; and I don't think it will translate to more than like two unique base game third party newcomers in the still-hypothetical Smash 6 (maybe three if there are some cuts, or maybe even just one again if they return a lot of vets). Of course, I expect several third parties to continue to hold demand throughout the base game speculation, but that hunger probably won't be fed quite as much as some expect (at least until another potential round of DLC), especially if a lot of the 3P speculation ends up being focused on which vets will even return instead of just dreaming on newcomers.
Another point is the diminishing return of third parties in a base game. Adding more 3P characters is stacking licensing costs but keeping a flat sale price for the final game, so each one you add has to be worth it. Given how little impact each individual character has on the overall sales, I don't think like five third party newcomers is going to make the game sell much more than one or two well-chosen picks which serve as the "headline" additions for the newest iteration.
Flatly false, in my estimation. The first party options are still very diverse, including top-flight fan faves like Isaac and Waluigi, other recognizable recurring characters like Dixie or Impa, a fresh litter of new Switch faves like Xeno3 or Ring Fit, and even a few legitimate franchise mascot-types like Tom Nook who lie in wait for their chance at the roster. Not to mention the stockpile of legacy characters that they can always dip into.
People love to go back to like the Brawl days and say the 1P newcomers back then were much "bigger," but even that game included many more niche choices like Ike, Lucas, R.O.B., Pit, Olimar, and Wolf. Like, Wii era Pit and Olimar are arguably even more lowkey additions than Isaac would be if he were to be added right now. Of course, there were also plenty of heavy-hitters like Diddy, Wario, MK, and Dedede, but only a few of these are ever really required to carry a given crop of newcomers. I could also say that if Melee had an actual proper roster instead of clonemania, then a couple of these could have hypothetically been in before Brawl, potentially making the newcomers tilt even more to the niche in that game. In any case, I don't see how the headline additions being, say, Tom Nook and Waluigi wouldn't be enough to excite casuals and hardcores alike.
As an extra note, I also think many people fail to realize how many hardcore and casual Smash fans would recognize and be satisfied with reveals for any number of overall less expected first parties, especially ones that have played roles as bosses or ATs in Smash before. See Marx, Porky, Lyn, Saki, etc. They might not be in the speculation limelight but would still garner a lot of positive reactions, just like less-expected third party characters such as Joker or Terry did.
