The left-wing argument is that businesses exist to maximise profit, so once you have the profit motive involved it'll eventually have to be paid for at point of use or not have it at all (assuming public funding is denuded).

This is a much of an article of faith as the right-wing argument that the profit motive means costs will go down and governments won't have to pay very much.

Both ends of the spectrum are ideological fantasies of course, but the NHS is caught in the middle.