We'll never know, but: I wonder what would happen if we moved the "medically necessary" requirement burden of proof from the doctor/patient to the insurer. So the insurer would be required to pay out a claim regardless of whether the insurer thought it was medically necessary, but their recourse could be to try to claw it back post-payment.
They'd most likely go bankrupt. There is already an incentive for them to spend on medical care due to the Medical Loss Ratio (MLR) which caps their profits on collected premiums. If you're saying they need to be forced to pay whatever invoice comes to them and start legal battles for each suspect case then yeah... that doesn't seem feasible.
1. pay out claims slowly
and/or
2. deny or downcode claims outright?
Really? That to me would imply that doctors/patients are submitting a huge amount of incorrect claims.