I mean yeah I guess they could streamline them like with Byleth and give them different sets of animations but at the same time they’re different enough from each other where I think different slots are justified, after all, Sakurai said if there’s one gameplay difference then the characters have to be separated, I don’t recall any of Byleth’s attacks having drastically different animations between alts like Lucina or Daisy do.
As for the Final Smash, it’s an interesting idea you bring up, and I definitely think Shinguji at least will get some assistance from Erica and Gemini, but at the same time Cherry Blossom Smite is her most iconic move and is something I really can’t see them not including, just like how Cloud had to have Omnislash or Joker had to have All-Out Attack it’s a move that’s basically synonymous with her so it would be strange seeing them not include it just to accommodate for alts, but hey you never really know for sure.
At the same time you do bring up really good points but I don’t know, I think it would be strange of them to introduce the concept of Echo Fighters, and then not do anything with it after release, especially since they’re allegedly very easy to produce ya know? That and with the remaining placeholder slots that could easily be filled by them, it all kind of falls into place for me personally, but hey, that’s just my line of thinking.