To add onto this, in addition to the governmental threat, another point that Cook is trying to make is that security is hard. It's hard for the government, which he backs up by citing breaches, and it's hard for Apple (which he omits, but remember those iCloud breaches?).
I completely agree with your point about future governments and unchecked power, but there is also the point that, if Apple creates it, there is the possibility that organizations (or even just normal people) besides these anointed three-letter agencies may also have the same power to access a good chunk of your entire digital life (and the locations of your friends and family who use Find My Friends) if you were to lose your phone.
I completely agree with your point about future governments and unchecked power, but there is also the point that, if Apple creates it, there is the possibility that organizations (or even just normal people) besides these anointed three-letter agencies may also have the same power to access a good chunk of your entire digital life (and the locations of your friends and family who use Find My Friends) if you were to lose your phone.