So I propose adding a new technicality into the mix: "Touch Wipe". In addition to recording fingerprints to unlock the phone, Apple/Android should add the feature where users record fingerprints to wipe (erase) the phone. For example, I may choose my thumb to unlock, and my forefinger to wipe.
Indeed, I may record only one digit to unlock, and all nine remaining digits to wipe. Or even, I may decide to record all 10 digits on both hands to wipe, and not use Touch ID at all to unlock (relying solely on the passcode).
This now presents the problem for the police. They can't force me to unlock the phone. They can't get around that by using my fingerprints, because they might inadvertently destroy evidence.
The legal system is resilient against legal trickery such as this. If think you've figured out a way to beat the system, then it's usually because you just don't understand the system well enough. But I think I've figured out how to beat this system, so I write this up so that lawyers can explain why I'm wrong.