All volunteers? Not quite. Though health care practitioners in single-payer countries do on average get paid less than in the US, they're far from being volunteers and they still get paid significantly more than an average professional.
Check out this comparison from 2009:
Also, I'd argue that disincentives and stagnation shouldn't be a real concern. Most well paid Doctors aren't going out of their way to develop new theories, treatment, etc... most of those things come from Doctors across the world and at home who have new and better ideas on how to best help their patients. Plus, nothing in a single payer system would limit those Doctors and others from being able to patent procedures, techniques, or new tech, which could then be sold for extra profit to themselves above and beyond salary. Nothing stops them from publishing. In fact, nothing really changes, except in the long run their school costs likely come down somewhat as well as their long term salary expectations, but not significantly.
It is recognized generally that western Europe and the US have the worlds best doctors and part of that is that those doctors are coming out of US medical schools.
So your position is that people become healthcare practitioners for the money, not because they have a drive to do that? Typically that is the opposite concept of most people with success in their field. Usually they go to do what they love and the money follows. The only time that doesn't seem to be true is with Wall Street types and sometimes lawyers... and probably most politicians.