I've imagined it, and you can imagine it benignly, with everyone orderly moving to different jobs and magically the jobs move to them, etc. In reality it would be upheaval, strife, rafts of people falling through the cracks, and in the end pain and resentment of the new system. Tons of people saying it wasn't worth it.

I don't think that's the reality either.

In reality, it would end up being just like the "doc fix" retreat on controlling Medicare costs. Remember that? In the late 1990's, Congress passed a law that would restrain the growth of health care spending by placing a limit on how much Medicare's payments could grow. There was a formula (the Sustainable Growth Rate or SGR) that was to be applied, and that would place an upper bound on reimbursement rates.

And then for a decade and a half, Congress would just not do it. Instead, there were a never-ending series of "doc fix" provisions that blocked the cuts. SGR never actually ended up limiting reimbursement rates for medical services in Medicare. Because, again, actually cutting reimbursement rates would have been very unpopular. They did that seventeen times, until they just capitulated and scrapped the whole SGR mechanism altogether.

medicareresources.org - What is the medicare doc fix legislation

Under single-payer, the medical industry basically turns into the defense contracting sector - but about eight times the size, both in dollars and employees. That's not a recipe for stringent Congressional cost-cutting and fiscal restraint.