An interesting approach to gently transition from the current pattern to Nostr.
But it greatly reduces Nostr's “one key, multiple platforms” advantage during the first exploratory sessions, since the user cannot easily test other applications without running the exit procedure. Or worse, start creating multiple accounts to overcome this problem, and he cannot merge them later, of course. I think a lot depends on which application the user starts with.
Note: after ejecting the login procedure should remeber the old emails (hashing them), so:
A) It's able to point the user to the new login procedure that uses the ncryptsec;
B) Refuses a new registration with the same email, avoiding confusion for the user with more accounts;