but why let them exist in the first place
Same reason why we let you exist in the first place. You're as much of a waste of space as anyone else on this planet. You have your own opinions, you express them. SJWs have their own opinions, they express them. Nobody is right, nobody is wrong.
Social Justice Warriors make it their life goal to unjustly defend other groups of people in their own self-righteous manner. However, if you're the one attacking the SJWs, you are actually the worse of the two.
Fascism is a terrible concept for that exact reason. There's so many different people in the world, so many different points of view. Strong becomes subjective, and so does weak. In the end you just have another group of self-righteous starfishs who will be out for blood.