Snowden for example joined nostr with a private that starts with "npub1sn0wdenkukak..."

So any impersonator who is trying to copy snowden's profile would have to invest time and money (power for mining) to find an npub which at least looks similar to be able fool people...

That only "works" if clients display the npub at all. That's why I included my vanity npub into my NIP05 verification, too.