Not an expert, but I've followed the growth of this word a little on-and-off.

Disambiguity can be important in a language. But it's complicated. Many times we use "he", "she", or "he/she", gender is not required. Back in the 1800's, the standard was to use "he" when gender was uncertain, unimportant, or ambiguous to a conversation. Obviously it had to do with the presumed defaultness of the male gender.

For a while, people toyed with "it" or "which". Honestly, my personal feeling is that it was the way insult could easily be taken (or given) with "it" that it died out.

"They" probably should not be used in cases a less ambiguous word is more appropriate... But that's when the bigots come out. In most cases, the most appropriate word to reference a person is that person's preferred pronoun if you know it, even if it's a genderfluid pronoun. Why? It's significantly more descriptive than "him" or "her". But these same people who consider "they" too general would break down to acknowledge any person having a gender identity different from their birth sex (and probably their genital birth sex for intersex folks, at that).

What all the offense is REALLY about is that they want to pretend some people are fiction, or subhuman. I think "it" would settle well with those folks. Which is why I'm glad that isn't a default.