My impression still is that microsoft should not weaken such updates because linux distro are insecure/outdated. Even for dual-boot this should be in place and, yes, kill access to the insecure systems.
It was not an update out of the blue, this was known to happen. I love Linux to my core, but if your bootloader is that old/insecure. You deserve this and you should not point a finger at microsoft in this case.