Originally Posted by Prestidigitweeze
The problem with presuming that an historical precedent for they trumps the need for a gender-neutral pronoun is the absence of its systematic adoption in earlier times.
An interesting point.
I think that it hasn't become more prevalent because of a systematic campaign against its use by English teachers over the past couple of centuries.
Now that it's more accepted that they (etc.) can
be used as a singular pronoun, their use will become far more common. And so become even more accepted.