[DiogenesKynikos]

> 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.