I get what you mean, but having nothing to do in singleplayer just makes it a worse game for those of us who just want to get a number of objectives done and then play it once a year when folks come over. Even having outfits, or a passport with stamps to unlock would be something to grab my interest.The trouble is, locking content unlocks to primarily single player modes is a absolutely terrible idea. Mario kart is at its very core a multiplayer focused game and tying anything to hours of playing a single player mode is a bad idea and should never happen again.
That said, MK Wii did go overboard with the unlock requirements, specially for characters like Bowser Jr, where you needed 3star rankings on top of the gold trophy.
I'm guessing what you suggest is that it works like Smash Ultimate and that characters get unlocked regardless of what mode you're playing so that the multiplayer folks are happy too, right?