In English, "they" has two main purposes: Theoreticals where sex doesn't matter, and as a gentle nudge to the person you're talking to that you don't really know the person you're talking about. That second one is completely lost when you just use "they" for everyone.
Though I haven't felt the need for this so far, nothing comparable in French, annoying to have to stop to say the equivalent of "he or she, btw?". I guess we could use iel now. Still sounds new and weird to most people but it might become familiar at some point.
I've seen people using -@ as a suffix in Spanish to cover both the -a and -o forms of gendered words. I have no idea how it's pronounced, and it's probably comically inconvenient in actual use, but I have to admit it looks mildly clever.
