Why did you jump to a 4 digit PIN instead of a 10 letter word? (which is still faster than the full 20 letter master password with many special symbols)
Is it because of the name PIN? So there is your simple answer of what problem Bitwarden needs to fix
Your mirroring fails precisely because it's a password, and it's pretty common to set minimum requirements, so you can't have 4 digit pin
Similarly with your first point - I don't think the master password has many special symbols, I'm just asking a question that illustrates the issue with the original faulty assumption
Is it because of the name PIN? So there is your simple answer of what problem Bitwarden needs to fix