Those firms have no say over which treatments are preferred on which patients in single-payer health systems. Actually, it is the exact opposite. When there is a single payer, they can haggle on behalf of the entire population so they have huge leverage over the treatment costs. The government can simply say "You either sell at the price that I want, or lose access to the entire market in this country.". That's why American drugs can be 10-100x more expensive than the rest of the world.

Their employees are here on this site too, downvoting preventative care and anything that grants health at a low cost to individuals. The corresponding gatekeeper organizations like the Endocrine Society and the The Journal of Clinical Endocrinology do their best to shamelessly disinform and seriously harm the people, e.g. via DOI 10.1210/clinem/dgae290. And don't even get me started wrt how corrupt the FDA is, serving the biomedical firms, not the people.