If a cooperative is inflexible, couldn't another cooperative out-compete it? Provided the market is competitive, this doesn't seem like a big problem.

One thing I have read is that cooperatives are less likely to grow into monopolizing behemoths because they approximately maximize profit in proportion to the number of workers rather than overall; they will only grow to the most efficient size. I can't remember where I read this, but it makes sense to me (although I'm not an economist.)