What made Chrom so highly speculated in Smash 4 was that he was the main character of Awakening, so people expected him to get in, which is what made Robin and Lucina getting in over him so surprising. He did get in eventually, but it just adds to what you said about the main protagonist not really being needed to represent a game all the time.
As someone who knows almost nothing about Fire Emblem I thought Robin was the main character because Robin was the player character, you even choose the gender and stuff, but apperently I am wrong.
Yeah, when Fire Emblem Fates was being advertised, they made a big deal about Corrin's role saying something along the lines of "this time, YOU are the main character."
As a massive Chrom fangirl it hurts my soul when the discussion of a hypothetical Smash reboot usually ends up excluding Chrom and how I usually end up agreeing with it. I was really upset when he was excluded from Smash 4 and didn't even get so much as an alt (Ike's palette swap with the Ylisse insignia doesn't count and you know it) and was so happy when he was confirmed as an echo for Ultimate. Seriously, my most wanted character was an echo lol. On the other side though, him just being an echo would make it super easy to just put him in the game as long as Roy (or even just Marth for that matter) are in future games they may as well have Chrom. Hell, even if he's just an alt for Marth that'd be okay by me too, even if I do prefer Chrom's playstyle.
I would prefer Robin in Smash 6 if they had to cut one of them just because he's the only character that represents the magic abilities and item-use/management mechanics of FE
What you think about Pikachu, Robin, Pyra/Mythra, Mio, and Sans is sorta what I think about Flowey except I consider Flowey the main character of Undertale
