You are wrong. Every BIOS company has to pay royalties to MS, and contain some MS code. BIOS does not just run "something", but has a lot of required hardware standards, like SMBIOS tables, ACPI, MasterBoot Records, etc. that have to be in firmwear or consistent hard drive sector. The fact another operating system can be loaded later, does not entirely replace all these firmware dependencies, vulnerabilities, etc. For example, when a new device is plugged into the USB port, it still has to have its EPROM interrogated by firmware and the device added to the SMBIOS table. Windows does not do that. All Windows knows about is what pops up in the registry.