Apple will most certainly NEVER update the bootloaders on iPhones.
The reason is that if the update screws at any point during the update, your iPhone will become a permanent brick that not even Apple will be able to unbrick.
The reson why it will become a permanent brick if the update fails is because a broken bootloader or malfunctioning one for whatever reason will not be able to start running the Baseband or OS, which is what runs the device.
Moreover, a bootlaoder update is risky and too costly is anything goes wrong, which will most certainly happen to many.