Couldn’t there be something equivalent to dns protocols that will fetch npub from username by storing such metadata in relays?
Discussion
nip05 maps email-like identifiers to npubs, but
1. Getting a nip05 isn't always obvious
2. People are not used to @ mentioning via nip05s, but maybe that should be more explicit in mentions?
3. nip05 has similar discoverability problems, and not all nip05s have embedded relay listings.