You’ve never used Linux?

Signed firmware just means you can prove a given key was used to sign something. Most Linux distributions sign their packages so you know one of the trusted keys from the maintainers was used to sign the packages (and yes, this includes firmware), which prevents a man-in-the-middle from modifying packages.

The only problem I have with signed firmware is if there’s no way to change the acceptable keys. Signing itself is an important security feature, its only problematic if the user can’t upload their own signed packages.