While you technically aren't wrong that DK and Diddy can represent Donkey Kong Country very well, that argument as a whole doesn't really work.
I mean, we only REALLY need Link, Zelda and Ganondorf for Zelda, and we only really need Peach, Mario, Bowser and Luigi for Mario.
You argue that Rosalina and Bowser Jr "Became so integral to the franchise" yet neither of these characters were even in Super Mario Odyssey. Those characters are literally just fluff for the roster. In the example of Dixie Kong, I'd argue that she's historically more "integral to the franchise" of DKC than Rosalina EVER was to Super Mario. Not that it matters, since that argument doesn't really work in the context of smash.
And if you want to argue "Rosalina got in Smash because she was integral to the Super Mario franchise at the time of Smash 4's development" then guess what? Bandana Dee is literally the 4th most "integeral" character to the Kirby franchise. If you have done your homework, you would know the character has had multiple SIGNIFICANT roles since smash 4. Similar thing for Dixie (although I can't really say the same about K. Rool) I will admit that Dixie wasn't in a major Donkey Kong Country game since Donkey Kong Country 3 on GBA, but we can say that she has STARRED as THE MAIN CHARACTER in HER OWN GAME. Literally only 2 other Donkey Kong Country characters share this role, and they are both in smash. Not to mention she is literally playable in the most recent DKC, a game that's getting a remake.
Although, that last paragraph doesn't matter, because the whole "The series doesn't need any more characters since the core ones are in already" argument is invalid in the first place.
Edit: Apologies for the rant, I just REALLY didn't like that argument. I'm going to go watch "Emporer's New Groove" now