I had certainly not heard of Michael Fassbender before First Class, and I had seen and enjoyed Inglourious Basterds; he didn't stand out though. But also, this actor playing Sunspot and both of the girls rumored to be up for Dani are new talent. They've balanced that well, and to be honest, a lot of moviegoers are not going to know who Anya Taylor Joy and Charlie Heaton are.
I think there are plenty of reasons to be upset about the casting of this movie (alongside even more reasons to be excited about the movie in general), but the actors playing Magik, Wolfsbane, and Cannonball (and hopefully Dani, but nothing is confirmed there) are not really part of that.