If Wii U user base grows to a decent size, it should get some well deserved sales. Mario Kart shouldn't compete directly with it, as it's purely arcade, while Project CARS should be a "simcade" closer to a sim than to an arcade racer (basically it should offer some optional driving simplifications on top of a quite faithful simulation, this option is often offered also in the beginner modes of even purer sims *), and with users contributions possibly enhancing its sim aspects more than arcade ones. So the problem won't be Mario's competition, but if it will have enough users that like not only arcade racers, but also more or less faithful racing sims.
* For example, even the very unforgiving GPL offers some beginner modes, with lower engine power, shorter races and damage disabled.
