If being a main character is a requirement, then Sakurai should just come out and say he's never adding another Zelda character.
Regardless, I don't think this is how he feels. Rosalina was five years old and had only appeared in four games when he had decided to add her in Smash (Galaxy games, MKWii, and WK7), and she got in before characters who had existed two, three, even four times as long as her.
Above all, I think moveset potential is what attracts Sakurai the most. If the timing was right, I think we probably would've gotten Midna and a Champion in Brawl and Ultimate respectively.
True, however I think that really just reflects that Sakurai considers more than any single bit of criteria for any given character. Sure, he could have added Waluigi or another Mario character, but Rosalina brings something that no other Nintendo character could at the time: being a puppet character. Similar to Ike, Sakurai was looking for a particular character archetype. It's clear no other character, at the time, could properly fit that mold.
Granted, Midna and the Champions could very easily have great potential. But I think what stops Sakurai from adding them is that, unlike Rosalina, their longevity is on a timer. They might be classic, likable characters down the line like Ghirahim, Midna, and Skull Kid have become, but they don't have that same presence Rosalina would end up having in Mario games. That presence being, in literally every spin-off from that point forward.
Do I think it's a good reason? Eh, kinda-sorta-not entirely. I think a character like Urbosa would be pretty cool, especially because I like the designs of the Champions. However, even as a Zelda fan, it doesn't bother me that they didn't get more characters. I'm more than ecstatic, personally, that they revamped each of the characters to an extent and added some of the amazing music from BOTW, and the complete and utter memery that is the Moon.
Obviously my tastes don't mirror that of all Zelda fans, but the fact that any of the trio are actually potentially
good had me ecstatic from the beginning.