Earlier quoted context omitted.
Unions can negotiate things other than salary, like health benefits, paid leave, policy issues, etc. (Though in practice you're correct: unions rarely exist for jobs with more individualized pay.)
It is worth mentioning that if the Senate bill version of Medicare for All becomes the law of the land, then previously employer-funded healthcare becomes one less thing for union leaders to have to negotiate with employers.
Although, universal health plans also mean employees, unionized or not, win the freedom to change jobs because they’re no longer tied to jobs they hate just to remain insured. I don’t think many people are considering or discussing this at large in the conversation about MfA. This will be a huge social and employment benefit for people with families/children, as well as those with existing—especially serious and costly—conditions. It could be something unions will temporarily grapple with in defending their importance.
I have no doubt, however, that something will replace healthcare at the negotiating table. Capital and labor are forever locked in conflict.