Here's a simple solution. Insurance HAS to pay if a doctor recommends it. This is what premiums are for. If insurance cannot pay, they can go out of business.
If a patient breaks a leg and a doctor recommends not experiencing gravity for a while, do we fly them to space?
I am using hyperbole to demonstrate a point: there are meaningful economic limits that must be created by some entity and enforced.
What is the value of a human life? The GDP of a nation is a hard upper limit. The total money a particular person in question has access to is the lower limit.
If you run an insurance company (or you run government run healthcare) you can't avoid answering that question with an actual real dollar amount.
At some point spending money on a person with a particular condition does mean that someone else with a different condition can't have money spent on them.
That's why it's important to spend money on research; to develop new technologies and techniques, and improve existing technologies and techniques; that can make cost of care lower.
That's why it's important to have standards of care and treatment, for all medical providers to follow.
Even a moment's thought about the power disparity between an insurer and an individual facing medical problems leads me to conclude your "simple solution" has little to recommend it.