My iPhone forces password entry after 5 failed attempts at TouchID unlock. If you can quickly thumb the sensor a few times, you can render fingerprint unlock impossible.
Exactly this. If they can force you to give them a fingerprint, but not a password, just reset the phone. You haven't destroyed anything, the data is still there, it's just inaccessible without the password.
One possible threat here could be access being gained another way (court-order to provide text key), then with the device unlock key known being required to try each finger in sequence to identify if you provided a genuine try to unlock the device. Could be seen as obstruction (IANAL).