We've trained Americans (and pretty much the rest of the world) to believe health care is a right, and therefore, should be free. Most people place value on their doctor's expertise at about $20/visit. When their co-pay rises above that, they stop coming in. If the doctor gets paid more than that via insurance, yippee! If not, who cares?
People don't want to pay for healthcare, they want someone else to foot the bill. A small % recognize the value and are willing to pay out-of-pocket. Most will not.
Many docs cannot go completely cash-only. If you take ER call, you have to accept what the ER sends you, and you cannot refuse to see someone for lack of ability to pay. So let's say you are cash-only and the ER sends you a BCBS pt. You tell the pt you will only see them for $300 and then they can get reimbursed from their insurance. They say they don't have it. You say then come back when you do. They have a complication from delayed care.
There is not a jury anywhere in this country who will be sympathetic to you. As far as the Average Joe is concerned, you should not even consider getting paid. You should just see patients and then drive your Bentley home. You are rich anyway, they reason, so now you can pay this poor sole $5M for your callousness.
The concierge practice with retainers is a nice model, but I'd be willing to be that less than 5% of Americans would pay for it.