> Law enforcement keeping hold of my phone for 3 days is simply not a realistic problem for me.

It's not a problem, until it suddenly is.

Especially with the glaring example unfolding right in front of our eyes.

Could be useful in a scenario where you won't be using your phone often and really want to maximize battery life.

A reboot will eat a lot more energy than a proper suspend.

Google locking features behind the closed source, proprietary Play Services is "more reason to move to AOSP"?

You don't need Play Services for this feature to work. The design is not proprietary or even hard to reverse-engineer.

The reason it is implemented in play services is likely because of how much easier it is for them to ship the feature to the most phones possible.