We are discussing what someone would use when writing about a hypothetical person.

And that changes it how? It’s insulting to misgender someone, though I can understand how you’d think that there’s no harm in insulting someone hypothetical.

I suggest you do some research on the history of language

Per your suggestion, “they” has been used to refer to a singular person since the 14th century. “He” is currently masculine-only. I apologize if you misunderstood my use of “never” to refer to things around the 18th and 19th century (when it apparently was considered bad to use “they” in the singular) when I presumed that there was an implicit limit to modern usage of English.