Microsoft is about to create an environment where attacking their own signed bootloader might become difficult because of the money and expertise they can throw at it.
If there would be no other signed bootloaders, then users would still need a manual approach for disabling secure boot.
Hence, an attacker would still find attractive to discover vulnerabilities in MS bootloader.
But...when you will get your bootloader signed, then you create an alternative, human-less, attack path, which is going to be less resistant than MS one.
As you have correctly argued, this would require signed kernels, and signed drivers.
Right!
But signed is different from invulnerable, and the money you can throw at it, along with the GPL and the need to find agreement in the Linux ecosystem, would still leave obscure drivers/kernel bugs that no one has ever thought.
Then you, instantly become the least resistance attack path, along with zillion of problems you can't solve without affecting users or compatibility,.. and most of all.. now.. you also take the blame of:
"Linux is the reason why Windows get hacked.
Recommendation:
Never install Linux on a system which is Win8 certified"
End of the story.

Wouldn't have been a wiser choice to battle this on a different ground?
My 2 cents.