The dubious idea is that eliminating private medical care systems would open up a world of research into treating very rare conditions with high R&D costs.

If this was true, why wouldn't all of the countries with socialized medicine be doing it already?

The US already was, and to since extent still does. Same in the UK and other parts of Europe. Government funds a lot of medical R&D.

Thank them for the fundamental research that lead to the COVID vaccine.