But you can disable the biometric logon automatically. Either screwing it up too many times (like touching the wrong finger) or by quick-tapping the power button a number of times - upon which it falls back to the stronger password.
If you're in a position where you know bad guys with guns are coming, you can just power the phone down.
If you know they're coming but don't have much time, or you're unsure, you just thwack the power button a few times.
If you don't know they're coming, you're kind of screwed, but then again you would be anyways.
If the bad guys with guns are coming and they want to decrypt your phone, they can beat it out of you - rubber hose decryption is remarkably effective.
If you're in a position where you know bad guys with guns are coming, you can just power the phone down.
If you know they're coming but don't have much time, or you're unsure, you just thwack the power button a few times.
If you don't know they're coming, you're kind of screwed, but then again you would be anyways.