Se can't have a functional alternative if we don't try and experiment dysfunctional ones and improve them. No system arises perfect. The argument that there is no alternative good enough is a tool to abot the creation of a good enough alternative through the improvement of not so good altemratoves.
We don't requite perfection from capitalism, but require from it's alternatives. Why? To block the possibility of a alternative, as all systems have problems, and initial experiments are problemsl ridden.