Medicaid is state specific, but I’ve never seen any state create a rule that limits physician availability if you can afford to pay cash. The physician may or may not have to sign some state form related to some nonsense in advance sure. Straight saying the government can ban access would be a good way to get voted out of office.
In many states, adoption of children comes with Medicaid for the child. You are saying that wealthy parents like Brad Pitt could take their bio children to concierge pediatricians, and then they go to a Medicaid clinic for their adopted children? There would be a media poo storm about how adopted children are forced to have substandard care in the family. As a parent, I’d be outraged if the state required me to treat my adopted child as lesser or just different than my bio child in any way. It costs me $20/month for my children to have access to a DPC family physician with private waiting areas. It doesn’t take a Brad Pitt to afford custom care, so he wouldn’t be the only one with a pitchfork.