Not accurate. Nostr identity is already separated from relays in NIP-98. Which does scale to billions of users.
Relays may get to a million active users though. We might get to test that assumption, time will tell.
Nostr has delegation already. Though some are trying deprecate it. There are a few different key strategies. I am using subkeys right now in my testing.
There may be others.
Nostr keys are fine, they have been battle tested in bitcoin taproot already, and being adjacent to bitcoin offers unique possibilities. Which is really the value prop of nostr.