You could argue the same thing with fighters, but both are very versatile genres and games like Metro show that regardless of how generic a shooter is it can still be very good. The problem with genres is that there are bound to be 'too many', just look at J-RPGs, there are more of them than there are FPS games and it's a genre that is still going strong. Same with western RPGs, there are more of those too. When you crack onto a fantastic formula there are bound to be tons of them, the only instance I've seen where a genre has been done to much and has turned into total trash is the horror film.