Prescriptive Avoidance of They for a Singular Antecedent

The British accept they for a singular antecedent, perhaps acknowledging that we English speakers of all nationalities use it regularly.  George Bernard Shaw uses it in Pygmalian when Henry Higgins explains Eliza's use of "to do them in."

 "To do someone in means to kill them."

Thackeray uses it:  "A person can't help their birth."

Lots of good writers and other educated people use it.

But Americans continue to be prescriptive, insisting that we use he/she, which is very awkward, and just when I'm thinking that there's no carry over between what we teach and what students learn, I get sentences like this:

          Despite the fact that volunteer may do a bad job, he or she is a good person who helps people in need without getting paid.

You can tell that we're working on concessional clauses, and this student needs to pay more attention to meaning as well as to number.  I can give the helpful hint (a generalization) that a count noun requires a before it or s after it  and offer the suggestion that they (!) use the plural:

          Despite the fact that volunteers may do a bad job, they are good people who...

But they're not helping, are they, if they're doing a bad job!  So meaning needs to be attended to first.
They can create meaning by specifying some volunteers and others...

 My point is that her sentence doesn't make sense, and she doesn't have the habit of using an article or the plural form.  But she has  mastered the he/she nonsense we ESL instructors teach.

If they for a singular antecedent is good enough for  GB Shaw and Thackeray (and Jane Austen uses it too), why isn't it good enough for ESL instructors and ESL students?

(Please note that I consider Shaw, Thackery, and Jane Austen to be with us in the present, so you don't need to write VT over the is on my paper.)