Umm. . . because even the systems strongest proponents admit it won't come anywhere near universal coverage? And because there are no cost controls?
If people won't accept "socialized medicine", then they will never have 100% coverage.
With this plan, by your estimates there will be 10's of millions more people covered by health insurance, which I do not think is a bad thing.
There are some cost controls involved. The costs that can be charged to consumers is limited, which will implicitly work it's way up the chain. But there is also nothing stopping them from implementing more directed cost saving measures as well in the future.
If you are going to be realistic, you are not going to have a single payer system at half the cost over night. Healthcare is approaching 1/5th of their entire GDP. Getting down to the levels of spending in other countries will also mean massive layoffs, which is the main bargaining chip that the insurance companies have. People want to get there, but it will take time.