Something is always on to handle power button, tender battery, as well as USB. NFC payment has to be compartmentalized for security, as a tangential.

At that point you might as well fuse those features into one companion computer and let it handle NFC payments under certain conditions, so people can take a safe ride home with a dead phone.

The iPhone continues to run various functions while turned off (including find-my-iPhone with BLE via the global, mesh network of iDevices)

Because not all of it is turned off.