Or stop using cell phones.

I'm going to have to study this more to see what the reasoning is, and whether it's sound. At first blush, I don't think that it is.

but in this case its your fingerprint and its being treated little more than as a key which is why if you or anyone else wants to really lock their device then dont use any face or fingerprint unlocking and instead use a pin and or password.

And in the future the phone will detect your stress level and ask if you are being coerced. It will know if you are forced to lie. Though this would be a nightmare if things escalated to torture. It could be like being handcuffed to a briefcase, but without the key.

Another tactic against criminals would be to use your pinky if things were safe, your index finger if you were being forced at gunpoint. The index finger one would lead to a generic, fake, user interface.

If used in combination though....

If the purpose of the unlock is to establish identity, then that would be correct; but if the purpose of the unlock is to read what is in the phone, without a warrant, that would be a violation of the 4th Amendment.

between being forced to cooperate in collecting evidence against you, and being compelled to testify against oneself?

Fifth Amendment is pretty weak if it doesn't prevent this tactic.

Police can't force you to answer questions, but they can compel you to hand over the keys to your safe.

Of course, it's trivially easy to prevent this if the police want in to your iPhone. Either restart/turn off the phone before the cops get it (after a restart your passcode is required because the passcode is what decrypts your phone), or if they do get it and try to force you to use your finger, attempt to unlock using 2 or 3 fingers you haven't enrolled. After 2-3 tries Touch ID is disabled and you're forced to enter the passcode.

the password is required when it's turned back on. I really have mixed feelings on this - obviously if the man really did attempt to strangle his girlfriend, I hope he is convicted, but with forced self-incrimination prohibited the onus should be on the authorities to retrieve whatever data they require. I understand the judge's argument in passing this ruling, but if the fingerprint is serving purely as a password in relation to the phone then I believe it should be subject to the same protections.

... the passcode was written down and found in the suspect's wallet or on the floor of the car?

That would be found evidence rather than forced self-incrimination.

...and while it is not exactly the same, it does seem close to the case of the fingerprint.

Then unlocking it could only be done with a password.

I like the idea someone said about a distress finger. Usually when you do the set up it asks you to use a particular finger. How hard would it be to set up a program to have a secondary finger scan for emergencies that would not only lock down your phone, but notify someone you care of where you are? I can't believe it would be that difficult.

Why not "use" another finger than the one used to set up access?

I'm trying, Officer, I'm trying! It just won't work!
Try another finger!!!

I mean, if you have ten fingers, would the police require you go through all ten?

And add a charge of false statements if you mislead them.

The right to remain silent should be exercised. And phone operating systems ought to evolve such that they don't directly give away the security method.