Water occurs in nature without human intervention. I don't think the same can be said for PFAS.

For a big chunk of the industrial age we've operated on the assumption that the global environment is big enough and capable enough to just eat or neutralize just about anything we dump into it. I think there is utility in defining a category of human-produced things that we have been able to determine the global environment can't eat or neutralize, and also that cause harm when people are exposed to them.

You could give this category a lot of potential names, but "forever chemicals" conveys the idea pretty effectively IMO.