You wouldn't be banning that... You'd be banning substances that have similar harmful environmental effects.
Let industry and the courts argue what exactly harms the environment or is too similar. But industry will think twice about using anything similar because to do so they'd have to take on a court battle to prove whatever they decide to use isn't harmful despite being similar.
And a blanket ban does not prevent you adding a carveout in the future if the next PFAS turns out to be a magical chemical capable of bending space and time.
The problem is that these PFAS chemicals do things that are useful, and banning any future useful inventions isn't nearly as desirable.