A phone screen unlock is not like passwords as used elsewhere. It has to be short; otherwise it is impractical. We know that short passwords don't have much ent

A phone screen unlock is not like passwords as used elsewhere. It has to be short; otherwise it is impractical. We know that short passwords don't have much entropy. We also know that we can examine the grease on the screen and make a good guess as to what the password is. If we consider phone unlocking mechanisms to be in a different "not fully secure, but at least practical" category, then I think it's perfectly acceptable to use a fingerprint as an unlock. Mitigation is possible too. For example, the phone could lock out and require a proper password if it detects tampering (which, AIUI from other comments, the iPhone does).