If we for a moment pretend biometrics are 100% perfect, secure, foolproof and able to determine if the fingerprint is provided from a live and willing judge. Just for the sake of the argument.
Are you planning to grant US Federal judges this power over all phones in the world? Isn't that a bit overreaching, and out of jurisdiction? Or only handsets sold in the US? (And what makes you think handsets wont be bought in Mexico, and resold in the US...)
How about other nations? Should every judge at the level determined by the individual nation be granted this? Would you like the commissars of Putin or Kim to have access to your phone at will?
And what happens when a judge is replaced?
And how long would it take before apps had a possibility whether or not to trust the builtin biometrics, for instance requiring a password or other form of authentication instead.
So in short. This would greatly reduce the basic security of the handsets. Add a lot of cost. And slightly inconvenience anyone who wants to keep something secret.