I wonder why they weren't using Public-Private crypto in the first place?
Perhaps because it would use up more space in the ROM? If so, I wonder what functionality they dropped from the ROM to add it now? I somehow doubt they made the ROM bigger - that would be very expensive at this stage of the chips lifecycle.
Mainly because it wouldn't help with this issue. The code signing used in devices like phones and game consoles is to prevent users from running unapproved software on the given hardware. The issue here is users taking the software and running it on unapproved hardware.
At some point, the software has to be decrypted on the physical device to run. The best you can ever do is put enough physical hoops in the way to make it impractical to defeat.
Perhaps because it would use up more space in the ROM? If so, I wonder what functionality they dropped from the ROM to add it now? I somehow doubt they made the ROM bigger - that would be very expensive at this stage of the chips lifecycle.