It is much quicker to understand they as a neutral instead of introducing new language and trying to disseminate that through textbooks. This way, there’s no need for any (or many) edits, we can just maintain existing grammar with new understanding.

English already has another form which refers to singular and plural: ‘you’. I assume that people who suddenly take umbrage are just kicking up a fuss for the sake of it, or simply didn’t stop to think about what they’ve been using all this time.