The idea of Mario Kart games is that they involve Mario characters, duh. Did Link and the Inklings change that? It's still called MARIO Kart, right? You still see MARIO characters front and center on the cover. The "core identity" didn't change a bit.
DId Link, Heihachi, Spawn, the female Spartan, Yoda, Darth Vader, Starkiller, Lloyd Irving, Ezio, or Devil Jin change the identity of Soul Calibur? Three of those characters are from comics and film.
Did the xenomorph, the predator, Jason Vorhees, Freddy Kruger, Leatherface or Kratos change the identity of Mortal Kombat? Not a single one of those guest characters are from fighting games... and outside of Kratos, games in general.
Actually, yes they did.
But in the case of Mario Kart at least, it was a good change in my opinion. Many people wanted it. The others, ehh... no.