I'm not a lawyer, so take this with a block of salt.
The 5th Amendment does not shield against valid search warrants. Nor does it shield against a subpoena. Giving up one's PW, IMO, would be part of one's duty to submit to a search warrant, just as unlocking and opening one's front door would be. In the case of a subpoena, giving one's phone PW is not testifying against oneself.
That's pretty much where I think this should fall.
Warrants shouldn't be handed out to go fishing, but if there is a crime and they believe there is data to support it on the device, then yea... you must surrender the keys.
I don't agree with it as part of regular infractions, traffic stops, etc... just like any other search.
If you are a criminal though, it may be worth 6 months in the stony lonesome to refuse...
I'm all for police powers such as investigative decryption, etc. as a basic idea... But I parted ways supporting it years ago. They can't do it without abusing it.
It doesn't matter to me that 95% of the time it's done right, it's done honestly and it's done for the right reasons. It's that 5% that makes it unacceptable.