You've kinda proven my point actually. There are various Impas, but then you give me a character that only had prominence once with everything else being minor rolls if they're even canon at all, and he could make for a really good moveset, but he's about as important as say, Zant (and you actually tried to pull the collection spin off where he didn't even make the base roster in a game dedicated to the series). I mean, you literally threw in an octorok. And in that same argument you call Mario "overstuffed" while saying legacy franchises are disrespected when...you don't get any more legacy than Mario. He's Mario. He is the king. If franchise merit is the benchmark, the series should have more.
Sounds more like you have some extreme bias considering characters of less importance, or even non characters at all, as having more value than literal protagonists because you think the series deserves it more. And, hey, not like I'm particularly fond of, say, Three Houses either, but the fact is that Zelda in particular has a very static roster (role wise, yes I know the Zeldas and Links are usually different) with some side characters, sure, but they aren't exactly stars of the show. I'm all for boosting both Zelda and Kirby's fighter count. Though I'd probably limit Zelda to adding an Impa and OoT/TP Link for his classic moveset as well as fixing Ganondorf who would essentially be a new fighter then with maybe some more tweeks to Toon and Young. Not saying you couldn't even add Skullkid (I've had ideas for a non clone Dark Meta Knight after all), but don't act like he deserves to be in it and FE stole your lunch.
Also, I never said they didn't have value.
And rotating cast does indeed have value as an argument. I'll use myself as an example. I am far more interesting in using Chrom, Lucina, and, well, one of the Robins than I am, well, pretty much any of the other main characters from a character standpoint. If the roster was just like...Marth and Ike, I wouldn't be too thrilled. Meanwhile, for the most part, if you check the box of a Link, Zelda, and Ganondorf, you've appeal to large swaths of the Zelda fanbase with anything else really being kinda niche by comparison. There is a reason HW went over so much more smoothly than FEW. FE is a much, much more fractured fanbase.
And, for your information, pretty sure TH was already out, and had been for a while by the time the Byleths were announced, so how are they any more promotional than anyone else? If any fighter was promotional, it was DQ Hero.