> 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.
Right I am probably being sloppy with my terminology. It's more notions that there is no such thing as "good grammar" is just tiresome. We can argue the rules are somewhat arbitrary, but that does not change how people might perceive you.