TBH, I think that with the "fixed cast" franchises like Mario, Kirby, and (sort of) Zelda, it's very easy to say who is and isn't important. We can't say FLUDD, Elfilin, and Skull Kid are important outside the games they appear in.
Sure, I think Mario from now on will get by on giving certain older characters their time to shine and/or promote them to spin-off regular, but does Zelda need a core main cast beyond the Triforce holders? Does Kirby need anything more than the perfect quartet of Kirby, Meta Knight, Dedede, and Bandana Waddle Dee?
Pokémon, Fire Emblem, Xenoblade, and other "rotating cast" franchises will continue to accrue important characters, though, but there's the issue of those characters falling off in importance when the new hotness comes around.