They is not singular. Replacing gendered pronouns with 'they' shits the flow of the sentence and creates ambiguity. There's a whole class of expressions that just stop working without a singular pronoun, hence the proposed 'ze' or whatever it is these days.
And what do you propose for all the languages where nouns and verbs and adjectives are gendered? In Russian, using gender-neutral phrasing when speaking about someone is an insult that implies that they're a thing rather than a person.