If it becomes this easy then Secure Boot just becomes Vista-era UAC. Sometimes making the security bypass an intentional act that requires some knowledge is a good thing. Most PC users, were their bootloader compromised and they saw such a screen on startup, would instantly press yes and forget about it within 5 minutes.

Not to say that having Microsoft as the custodian of the keys preloaded on all PCs is the optimal solution, but I don't think a token yes/no to add any random key on boot is a good idea either.