You're right! Cooperatives have a fundamentally different structure that makes the typical metrics of return to non-employee shareholders hard to use or make sense of.

Fortunately, I think this is addressable. Cooperatives still have owners, be they employees or customers. The structure works by merging two of the three legs you sketch out. For worker-owned cooperatives, it should still be possible to evaluate how effectively they return profits to owners. You just have to evaluate it a little differently, perhaps by classing worker pay as dividends (or maybe the premium over non-coop competition).

Plus, you can look at things like their offerings and the cost or quality thereof. I don't know anyone who goes to Rainbow Grocery Coop for reasonable prices.

To recap, you're absolutely right. Coops are not optimizing for returning profits to uninvolved investor/owners. Instead, they are optimizing for returning profits to actively involved who act as one of the active groups above. As a result, I would think that metrics for evaluating a business could still be calculated and examined for a enterprise. After all, cooperative enterprises are still businesses.