I agree almost completely. It's just that my guess is that nobody actually cares about usernames either, just the few people who can't use a phone for <reasons>. So I'm thinking they're already kinda in the realm of building out this feature for nobody which is why I was suggesting something more wallet-like like generating 32bytes of entropy and showing users the mnemonic representation and telling them not to lose it (which is familiar, despite being a terrible UX, at least). Perhaps I'm underestimating how many people actually would use a username instead of their phone number in which case I think your 100% spot on.