Worker owned co-ops are 100% compatible with capitalist regimes, since they're just people freely consenting to a work/ownership arrangement. I think they're cool experiments, if anything.

With that said: if they're so much better than traditional firms, they why haven't they taken over the world? That is, if they were so good, greedy shareholders and investors all over the world would demand that their firms restructure into Cooperatives, and firms that weren't Cooperatives would tend to lose out in the marketplace (being beaten by the more efficient Cooperatives). But we don't see this happening.

This seems like strong evidence to me that they don't offer a superior arrangement (at the firm/system level) for producing goods & services more efficiently.