Earlier quoted context omitted.
If you can identify a person, in any way, they are no longer private. This is not necessarily applicable. There's cryptography from 20 years ago (e.g. the work of Stefan Brands) that can show that someone has a World ID without revealing which ID it is. If no "username" is ever revealed then it can't be linked to anything.
If the idea is to make "World ID" universal, in that everyone is going to have one, what does attesting that someone has one means? Nothing. Virtually nothing. Everyone either has one or can get one, to attest that they do means nothing of value.
Maybe you can find a few thousand on the black market, but that's going to add up fast.