> Dangerous tools often have safety guards built-in to the design
A phone is not, usually, a dangerous tool. Unless you can make it explode :)
> For most people, iOS limiting the damage when they get compromised is a huge plus.
Then just dont unlock. The unlock would need to be done from inside the OS, by the user, not by external actor.
> Now, consider what that means for something like unlocking the bootloader. Allowing that means undetectable malware, potentially irrecoverable. There's not really a good way around that and even if you try to inform people at the time they enable it that doesn't help if it's done when the device is out of their control (abusive partner, boss, police, etc.)
If you have an abusive partner, boss, etc. you're already at risk because they will keep looking at your phone anyway and you're in no position to refuse. A way better solution is to have a burner phone they don't know about.
Agreed with the police point, but I think after getting your phone from police you could just clean install iOS again to make sure nothing unwanted is there.
P.S. On 2nd hand market, you could just as easily reinstall stock iOS once you buy a new phone to prevent buying 2nd hand unlocked iPhone with malware.