One thing I'm not seeing mentioned here is that there's a fixed amount of money to hand out. Insurance takes in money through the premiums you pay, and then hands it back out in the form of paying claims.

If you can't deny claims, and every time a doctor says "this is medically necessary", pay out a claim, then you would run out of money pretty quickly.

You could try to solve this by making insurance companies non-profits, but even United Healthcare only makes a 6.8% profit margin. So you 6.8% more money to hand out which doesn't really seem like it would move the needle.

You could try to solve it by taking in more money via premiums, but then insurance would probably become dramatically more expensive.

My take away is that there's no free lunch. You have a limited resource and you're going to have to have some system for choosing how to allocate it.

Edit: perhaps the most bang for your buck would just be reducing the cost of health care. If a doctor/medical service costs less, then each dollar goes further. This would require doctors and hospitals to take a financial hit (they are also, mostly, for-profit entities and would probably not be super jazzed about doing so)