If the facts on the ground were that it was actually possible to get single payer passed, I think those questions would have been easily answerable. It just wasn't tactically necessary to answer them in the context of what was achievable because of one horrendous (sorry Lieberman, but you really blew it) person.
The statement was clearly entirely political, of course.
"What will happen to them" doesn't have to mean preserving a broken system, it just has to mean caring humanely for those affected.
Also according to that article, former coal miners are more open to retraining where other industries have jobs readily available and/or where coal has no credible chance of comeback in that area.
None of that contradicts my suggestion as applied to the health insurance industry, which isn't really part of any cultural identity, and which would clearly not be about to rebound quickly in a single-payer world.
Retraining could even be pretty quick if they retrained to other forms of insurance, especially life insurance (which still cares about individual health) but there are many other kinds too.
A significant reduction in overhead would free significant funds into the hands of the consumers, who would employ them to improve their lives, generating economic growth and demand in other economic areas that would pick up the slack. Granted, it's not comfortable for the workers made redundant, hence the tendency to band together as a special interest group and put political pressure to keep things inefficient (that's all assuming single payer is indeed an efficiency improvement, an entirely separate discussion).