Insurance professional here.

Quite a few health insurance companies have so much money that if the health insurance gig runs dry, they can just become investment firms.

Some specialized personnel like actuaries and underwriters may be laid off, but finding a job in a different insurance market is not a huge deal.

Under proper scrutiny, a single non-profit health insurance organization for the country should be able to provide lower cost policies than a private market, simple due to proprietary administration. Each insurer spends gobs of money knowing how to manage risk portfolios, budget for losses, etc., and these efforts are done hundreds of times in parallel across the hundreds of insurers. Having one insurer should in theory reduce these market wide costs significantly, thus reducing market prices.

Practically speaking, converting a highly bureacratic process like insurance into a government function is a sure recipe for drastically reduced efficiency, too. So it's a double edged sword.