Do you think founders should retain ownership of the companies they create?

Fred Smith’s wealth comes from the notional value of his ownership of a small fraction of an enormously successful and societally useful business. It’s not as if he has a bank account with billions of dollars in profits paid as salary in it, which you seem to believe.

On what basis do you think he should have been compelled to sell his stake in the company he created?

Yes, you’re correct that he didn’t earn a billion dollars. He created a company that investors value at $60B, and his remaining 6% share of it is worth $4B. That $60B was not taken from anyone. It was created - out of thin air - by organizing workers in a way that the world needed.