I also wonder if a weak link could pose a threat. Say, Attacker knows User is a member of Site X, but Site X fails to implement checks and/or rate limits prope

I also wonder if a weak link could pose a threat. Say, Attacker knows User is a member of Site X, but Site X fails to implement checks and/or rate limits properly. Attacker brute forces master password by spamming the login endpoints. Extrapolating: a database leak of any site means the attacked can locally brute force master passwords of all users? Currently, such a leak means a subset of users who reuse passwords are in trouble. Widespread deployment of this kind of scheme would take away choice, and put everyone in trouble. (Unless you start using different master passwords to create bubbles, which means we're back to square one.)