Let us say we passed a law banning some people from the shoe trade. In particular, we would ban all people with last names starting with the letters A-D.
How much would this matter? Surely the E-Z people are numerous enough to enforce a perfectly competitive solution, or as close to that solution as we would have come anyway. But what if Mr. Brown would have been the Steve Jobs or the Bill Gates of the shoe trade?
This is a demarcation question for your vision of economics. What is the chance that individual agents will matter for final outcomes, or do good rules suffice to set things right?
If you think the A-D ban would matter, must you also believe in increasing returns to scale?
These queries are not unrelated to whether Wal-mart should be allowed to enter commercial banking.