[Not15CharsLong]

"People stopping stealing iPhones" is not going to happen. And the current Apple policy of "well, enter your AppleID credentials to unlock this phone" deals with that pretty effectively.

What we're talking about here, is disabling that possibility once the fingerprint sensor (OK), screen (huh) or battery (double huh) has been replaced.

In other words: after which "repairs" exactly does Apple get to disable a device? Some of these would enable fencing, others definitely not... TL;DR: yeah, difficult!