Remember your high school chemistry class
Yes I do. I also remember my college chemistry classes. And my work in an industry R&D lab evaluating potential replacements for a fluorinated compound.
What do you think they are going to use instead of fluorine?
Something that's not as good, but good enough. See leaded vs unleaded gasoline for a historical example of industry reacting to regulation. It'll of course take time and money, and there may be limited use cases where there aren't any conceivable replacements, but in a lot of cases these compounds are used as a catch-all because they work so well.
PFAS are used in so many forms (solvents, polymers, etc.) that I think the replacement will be very dependent on the specific use case (and potentially other regulations on alternatives, particularly for solvents). I'm not knowledgeable about every field these compounds are used in and for privacy/NDA purposes I can't talk about the specifics of the ones I worked with.