+20s already have exclusive rewards in dungeon teleports that are far more useful even post SL than "just another mount" will ever be.
KSM is not equivalent of a gladiator mount. It's equivalent of the vicious mounts like the current frog. It's basically a participation trophy and that's fine.
M+ even has a gladiator level title reward for the 0.5% highest rated at the end of a season.