“English” way way back was a gendered language across the board, like so many other modern languages still are. The reason modern English is a mostly non-gendered language is because when the Anglo-Saxons and the Danes (Vikings) were in constant contact with each other, the grammatical gender and inflections were not uniformly aligned between the two languages. So early English speakers ended up dropping almost all gender and inflection in their language. Gendered pronouns are some of the oldest words in English, and they’re still hanging on. This kind of makes sense, because we are, of course, biological animals, and as such it is very important for us to be aware of what pairings might produce offspring.

There are still a few things that hold on: some pronouns, and a few different inflections, like “children” instead of “child[e]s”. In English, the “singluar they” has been used since at least early Modern English; Shakespeare used it, for example. It’s usually not effective to try and dictate new words by fiat, especially when such words are intended to replace very old core words in the language. I see the “singluar they” being used much more easily and naturally now than it was when I was younger, and I expect that trend to continue. Eventually, gendered pronouns in English will sound “old-timey”.