What a weird distinction to make that it’s talking about an abstract gendered person rather than concrete. I don’t know why the grammar would make that distinction (nor do I think it does).

Then you're gonna be absolutely gobsmacked by the other grammatical distinctions that exist across the world's languages.

It doesn't matter if you know why the grammar would make that distinction or not - the distinction exists, and is widely accepted in the linguistic literature (as cited above) whether you think it does or not.

The argument that is almost always made is that “they can’t be singular.”

I'm not sure what that has to do with our conversation, since I've never made that claim (and neither did Thymos). If that's what you're basing your argument on here, then that's a pretty egregious strawman of my position.

Sure, maybe it historically hasn’t been used for a particular subject, but that’s a fairly minor grammatical shift.

And yet it exists nonetheless, rendering your "correction" of my original comment (and your "correction" of Thymos's comments in the other thread, for that matter) inaccurate and misleading.

If we’re going to argue that’s wrong because it isn’t historically accepted then we probably need to speak a totally different version of English than we do because it has made much larger shifts than that in the past.

I haven't argued that anything is "wrong" other than your description of the historical use of English pronouns. Linguistics is descriptive, not normative, which means that the historical facts of English have no bearing whatsoever on what we "probably need" to do.