- worker cooperatives, and
- quality and affordability
I don't know whether worker cooperatives are more or less likely than a median business to generate negative externalities, so I won't comment on that part.
I wouldn't call Rainbow Grocery 'affordable'. It's been a long time since I bought anything there, but I recall it being much more expensive than every single chain supermarket (not just the lower end ones).
King Arthur and Bob's Red Mill are not 'worker cooperatives' as far as I can tell. They both have ESOPs (Employee Stock Ownership Plans), but I don't see anything suggesting they're run in a democratic (one employee = one vote) fashion.