Maybe some day they'll unlock password managers of their own

from here

I bet you thought that was going to go in a different direction.

But seriously, the master password is meant to be better than all the passwords stored in the password manager. If it's weaker than the stored passwords then realistically that weakness transfers to the stronger passwords stored inside, because the weak password can be broken and reveal the supposedly stronger passwords.

The stored passwords aren't going to get stronger over time, though. They will never surpass the master..