All games need to progress in one way or another, and by adding bikes and stunts to the game, it is given more variety, it is given individuality among its predecessors, if you want Super Mario Kart then go play Super Mario Kart.
No one is forcing you to use the motorbikes and the stunts seem as though they'll be one-button-presses when certain conditions are met; like launching your vehicle into the air, it's hardly radical changes. Though we have no idea what the reward system is yet for performing stunts so it could maybe fuck the gameplay up in one sense.
Although Mario Kart hasn't had balanced gameplay since the SNES original. Ever since the blue shell was invented and shit players are given weapon advantages, and now with everyone moaning about snaking Mario Kart's multiplayer is evidently very broken.
Still this doesn't mean it can't be fun.