In my experience, everyone who complains about the use of the singular "they" uses it themselves all the time when they're not thinking about it.
The reason why there's any debate at all about the singular they is not because it's new and strange. It's because beginning in the mid-18th century, influential grammar textbooks started discouraging its use and advocating "he" in its place. Many generations of kids have grown up being told in school that the singular "they" is wrong, but despite that, it has remained a very standard part of spoken English.
Really, are you sure singular they was in widespread intemperate use, like today, prior to these influential Victorian grammarians?
OK, but they were influential, so they influenced the 1850s and subsequent decades, making this usage currently new and strange, because for a century or more people used he instead. Why deny that? To persuade them with the implication "we never got accustomed to saying he, turns out you didn't ever speak this way, it was just an illusion"?
I'm not sure what matters in persuading people to speak differently, but saying that a term is being revived, rather than being a complete neologism, is ... admittedly a little bit persuasive, but it doesn't much help with the glaring issue that it's still a major change from what we're used to: and there are additional valid complaints, firstly that it removes information, and secondly that it's used less sparingly than it was in the past. It's now commonly written, in formal texts where clarity matters.
This was for clarity in the phrasing of legislation.
I've picked up a rumor that this 1652 book encouraged the use of he in gender neutral contexts: https://archive.org/details/bim_early-english-books-1641-170... but I can't find where. It might just be an exaggeration based on the part where it says "The Maſculine is more worthy than the Feminine, and the Feminine is more worthy than the Neuter." But there's no doubt that the 17th century, never mind the 19th, was stuffed with sexist bastards in influential positions. So what's the use in pointing at the past, or even at the present, to say that some of the time they used they? Fundamentally you still have to argue for why, or why not.
> OK, but they were influential, so they influenced the 1850s and subsequent decades, making this usage currently new and strange, because for a century or more people used he instead. Why deny that?
They only succeeded in influencing formal writing. Singular "they" continued to be a completely normal and heavily used part of spoken English.
> but saying that a term is being revived, rather than being a complete neologism
It's only being "revived" in formal writing. It is style guides that are changing, not the way that normal people speak.
> there are additional valid complaints, firstly that it removes information
It allows you to not specify that information. Sometimes you genuinely have no idea what gender the person you're talking about is. "Someone is knocking on the door. I have no idea who they are."
> Fundamentally you still have to argue for why, or why not.
The argument is that style guides and grammarians artificially banned people from using a completely regular pronoun in formal writing, and that the alternative they offered (gender-neutral "he") is extremely awkward. We already use this pronoun this way in spoken English. We should be able to write it too.
The reason why there's any debate at all about the singular they is not because it's new and strange. It's because beginning in the mid-18th century, influential grammar textbooks started discouraging its use and advocating "he" in its place. Many generations of kids have grown up being told in school that the singular "they" is wrong, but despite that, it has remained a very standard part of spoken English.