A commenter asks, “Why is it OK to prescribe, and indeed proscribe, vocabulary, but not syntax?”
My sense is that descriptivists — and certainly this descriptivist — think that the guide to vocabulary is usage: An English word means what English speakers use it to mean. Dictionaries that reflect a word’s meaning also tend to preserve that meaning, because they tend to lead people who consult the dictionary to use the word the same way.
Consider an example: Many English speakers use “presently” to mean “at present” and many use it to mean “in the near future.” I see no basis for saying that one or the other usage is “wrong.” One or both usages may be dangerous: People who want to avoid puzzling some listeners should probably avoid the “in the near future” usage, and people who want to avoid alienating some listeners may want to avoid the “at present” usage. What’s more, the danger may be unnecessary: One can usually just say “now” or “soon,” as the case may be, instead. But while there may be reason to avoid one or both meanings, I see no reason for calling either incorrect.
Of course, this doesn’t mean that anything goes: If I use “presently” to mean “with presents,” as in “I’m coming to your birthday party presently,” I’ve made a mistake, because I’ve departed from the norms of actual usage. Likewise if I write the word as “presenly,” or pronounce it with the emphasis on the “sent.”
But to a descriptivist, the test is actual usage. If some self-appointed language policeman insists that “presently” can only mean “in the near future,” the descriptivist will demand to see a warrant. And since the descriptivist respects no warrant other than the grown order of the English language as it has in fact grown, the descriptivist will ignore the policeman’s complaint as being the policeman’s private prejudice rather than an obligatory language rule.