I'm not proposing that all guardrail responsibilities be shifted to the insurer. Just the "medically necessary" provision.

Doctors would still have a Duty to Code Services Accurately and a Duty to Maintain the Medical Record (which would clearly enable an insurer to prove a non-medically necessary therapies). There would be plenty plenty of evidence for an insurer to immediately respond.

So claims could be rejected on the basis of failing to code accurately or lack of record.