This is a non-solution if most of the names I like are non-ascii.

I would like to use emojis in my username. But as a programmer, I'd like to keep a system as simple as possible. If there were a trillion humans using my service Unicode with emojis would be a useful thing for unique usernames. But in the current scenario where the biggest services have users in the low billions, ascii is the most efficient and simple thing to do. You can always romanize Hindi or whatever language you want to use. The latin alphabet is unusually good at that. IMO it's not worth to introduce the complexity Unicode brings into something like usernames where the slightest problems can lead to stolen identities etc.