I mean, Roy got in Melee while his game wasn't even release and FE was pretty much unknown outside Japan at the time.
In other words, there was always some characters who were choosed for promotions purpose, rather to their game/own merits, ever since Melee atleast.
Soooo really, that "magic" you had pre-Corrin was never truly there, and honestly, I think it's better to just accept some characters are chosen for reasons we wouldn't necessary accept or see come in. As long as the character add something new and/or fun to Smash, they can have a place to the roster ( retro pick like Ice Climbers or Duck Hunt didn't got picked because they're popular or promote stuff, for exemple), atleast, that's how I see it.
Note tho that I'm not a fan of Corrin or especially Byleth lately, buuuut them being promotion pick are just minors problems in a list of others problems I got with them.