Maybe you are too young for this but viruses modifying boot partitions was a big thing back then. It is simply impossible to inject some code without finding an exploit in UEFI with Secure Boot or somehow exploiting the kernel. It is still possible to do this kind of hack but it is 2 orders of magnitude harder.

No, boot sector viruses were not a big thing, especially after DOS times. They existed, but they weren't the worst problem at any time.