The hardcore anti-prescriptivism among linguists does drive me a bit nuts as well.
Languages can and do alter because of peoples prescriptivist ideas. They're not just arbitrary rivers of sound changes that people cannot control. English is still full of Inkwell terms, for example. And in my own lifetime I have seen a lot of linguistic changes basically proscribed that everyone falls into line with (a less controversial/political one: no one in NZ called association football "football" at the turn of the century. We all called it "soccer". Then the sporting bodies and media changed what they called it and everyone around me changed it too. "football" used to unambiguously mean "rugby football").
> Languages can and do alter because of peoples prescriptivist ideas.
You are right, but that comes also from a descriptivist perspective. And a linguist would study what sort of prescriptions stick and what sort don't.
When linguists say they aren't prescriptivists, they don't say prescriptivism doesn't work, they just say their job is not about deciding whether to say football or soccer.