It isn't enough to simply look at the world as is. You must consider the world with universal deployment of kill switches and fully understand likely consequences as much as possible.
Stolen phones can be taken apart and sold for parts... Thieves doing this may well end up making more money than phone as a whole can be sold in an underground market.
If users have ability to opt-out then anyone taking phones by force could demand victim "opt out" putting owner in increased risk of harm v. lift 'n dash encounter lasting seconds. Further thieves could demand credentials to your online account linked to the phone and lock you out of it.
As they say "the road to hell is paved with good intentions"..
There are technical solutions that could work such as a "fused" opt-in where physical device can never be opted out after an opt-in without mainboard replacement or some kind of secondary duress password to covertly signal theft... yet it seems obvious nobody is going to implement fuses and secondary passwords.
The argument that calling the carrier/police is not enough seems to hinge exclusively on the notion of phones sold overseas out of reach of carriers...
This as far as I can see means anything that would actually work while not putting victim at increased risk is also by necessity as oppressive as hell. Users should NOT have the means to lock their devices themselves as it puts them at increased risk of harm and to be effective it must either work OOB of both normal cell network/IP Internet or implement a heartbeat/watchdog with a central server to continuously prove continued availability which is one massive single point of disaster.
The OOB signal could be some kind of special backhauled PDP anchored to US carrier? I don't know enough to even guess how it might be implemented or if it is even possible.
As most technical solutions to political and or social problem I'm drawing a blank imagining a scenario whereby kill switches make a positive contribution to the world.
If people really want to cut down on theft maybe they should use common sense when wielding expensive toys in public.... or ... ah... gulp... um... ... a... device vendors could always .... ...u... know... make them cost less.