I agree with your broader point (the inherent inefficiencies of market/competitive healthcare are subsidising a large number of jobs and so a large part of the economy) ...
but you aren't wiping off 20% of GDP overnight.
The number of doctors, nurses, cleaning and catering staff, ambulance drivers etc should be unaffected. The number of administrators will be reduced slightly. The number of billing staff will be reduced drastically. And a large part of insurance jobs will go away, with some absorption in the govt bureacracy (who will need some billing specialists and some acturial predictions).
There are actually calculations of what savings this would result in, and from memory it was about 10-15% of healthcare spending, so about 2-3% of GDP. Which is a lot! But a lot more manageable than 20%. And there are plans for managing the transition to efficiency, like increasing eligibility in a stepwise way based on age, wage subsidies for a year, etc.