Do we really need UEFI? I believe that there is simpler ways to achieve this. For example:

a. User turn ON a switch to enter "OS Install mode"

b. User install the OS of his choice. During installation the installer read a motherboard specific key from the BIOS and sign his bootloader/kernel/drivers using that key.

c. User turn OFF the "OS installation switch". System is save (at least as long as the OS in choice is not a Redmnond one)

Is there any fundamental problems with this approach?