You've misunderstood the meaning of two factor authentication. If you remove one factor (by trivialising it), then you no longer have two factor authentication, you have one factor authentication (just with a different form of factor - something you have/are, instead of something you know).
You imply that the benefit of 2FA is the strength of the second factor. If that were the case, then you don't need 2FA - simply implement the second factor by itself. No, the benefit of 2FA is the redundancy of having two factors at the same time.
If a hacker breaks one of the factors, you are still protected by the other. E.g. if your internet banking security device gets stolen, you are safe because you still have a password. Alternatively, if someone sees you type your password, you are safe because you still have your device. You've reduced the attack vector to scenarios where the attacker manages to accomplish both tasks.
If you trivialise one of the factors, you might as well not bother with the scheme in the first place.