The problem isn't arbitrarily keeping the customer away from the devs and replacing them with a PO. The problem is the customer is fucking busy doing business and supporting two other software initiatives. They don't have time. The devs can never get their hands on the customers time in enough quantity to be truly effective.

The PO is an always available customer.

And here's the wonderful thing about agile - if you happen to have an always available customer, you don't need a product owner