Diddy Kong is far from old school! Let me put it to you this way: Falco and Wolf were completely absent during the whole story of StarFox Adventures except for the very end of the game, Krystal never missed a StarFox game ever since then. (And no, StarFox 64 3D doesn't count, because it's a REMAKE)
So because they weren't main characters in what is considered to be one of the most controversial games of the series, that suddenly makes Krystal more important than them?
Yeah... No...
Falco has been in every single game of the series, not counting Melee and Brawl in this.
6 games + Star Fox 64 3D. Most recent appearance in Star Fox Command, 2006, if you don't count Star Fox 64 3D.
Wolf, not even counting his veteranship in Brawl, has appeared in Star Fox 2, Star Fox 64 (and 3D since you apparently don't count that as a separate game), Star Fox Assault, and Star Fox Command.
4 games + Star Fox 64 3D. Again, most recent appearance in Star Fox Command, 2006.
Krystal has appeared in Star Fox Adventures, Star Fox Assault, and Star Fox Command.
3 games. Most recent appearance? Star Fox Command, 2006, just like Falco and Wolf.
Either way you look at it, Falco and Wolf have been in more games than Krystal while still keeping up with her. They have had more opportunities to be major characters, in other words. And hell, if you're saying that we're basing this off of characters who have consistently appeared throughout the series, why not Slippy? He's literally been prominent in
every game of the series, including as a character in the same game Krystal debuted in. He's never been absent since. He should be in instead Krystal if that's the road of logic we're going after.
Being "old school" means nothing, in other words, when your shining thesis for that argument is one game that came out over a decade ago.
That is old school, my friend, not Falco and Wolf. So if you're going to say Krystal could easily replace Falco or Wolf in being able to represent an entire series, you're going to have to give a lot more than just say "She was important in Star Fox Adventures!".