In this case "they" has priority. We've been using it as a singular pronoun for longer than people have used X as a variable. Chaucer used singular "they" in the 14th century; X as a variable is generally credited to Descartes in the 16th.
I don't really have a strong opinion on this new sense of the word, but I do really dislike falsely involving Chaucer when pushing it.
Edit: This is the sort of disingenuous argumentation I'm talking about. Every single usage I saw skimming through did not refer to a single, known person:
https://www.oed.com/view/Entry/200700#eid18519864
The word evolves, as all words evolve. But there's no point in saying that this specific distinction means that Chaucer wouldn't have recognized the use and instantly figured it out. Chaucer used they as a singular noun, and it makes perfect sense to apply that to specific individuals. It doesn't make sense to say that because Chaucer didn't use the word in precisely this fashion, it invalidates the very similar senses in which it was used.
I'm honestly not sure what your point is in your second paragraph. My point is that Chaucer did not use "they" in the sense you're talking about. You're saying that people are now trying to use it in a very similar sense to how Chaucer used it. So we agree?
I don't disagree that it's a very similar sense, but that doesn't mean it's identical. So instead of invoking Chaucer, just say "it's close enough already, so let's just use it"
That's been said many times. There are people who, nonetheless, say it's incompatible with previous usage. Such people are, demonstrably, completely fucking stupid. That's what the Chaucer is there for. Not that it helps, because nothing helps against complete fucking stupidity.
Regardless, I appreciate the straightforwardness of your first paragraph.
https://en.wikipedia.org/wiki/Recency_illusion
It has a few references there that I have not confirmed myself but it does list a reference to Shakespeare.
"There's not a man I meet but doth salute me / As if I were their well-acquainted friend"
For example: "Here is Bob, a builder. They are moving here to find work in construction."
I'm mostly indifferent about pronouns, but I wouldn't call someone "they" also because it's too much like the royal "we". Unless they are royalty of course.