Super Smash Brothers Ultimate - Sakurai reaffirms: Don't even need to be the main character of your game to get a spot

Don't remember that was a point in the matter though.
I think he just commented it on the Min-Min video who among the other Arms characters was considered a main character in their own context and there was a similar argument with Piranha Plant.
Anyways the avatar from Awakening kinda shares the spotlight with Chrom really so it wasn't as far away as not putting a main character in that case and Lucina just was an echo.

Sakurai: