> Bootloader unlock is under user control by definition - it won't happen unless you're physically interacting with the device.

Root exploits more common, but bootloader exploits aren't unknown. String them both together and you can remotely root a device, unlock the bootloader, install another OS, and relock the bootloader. All of this without removing apps or data; do it properly (custom recovery for flashing the new OS) and all the user knows is that the device wasn't working for a while (until they notice the giant new notification Google services puts up now, but some users don't even know what notifications are and either won't notice or will ignore it) https://github.com/segv11/boot-unlocker