I do wonder if we're somehow talking past one another. We appear to agree that there's no fundamental reason to trust firmware - it's possible that it's deliberately backdoored, and it's almost certainly buggy in exploitable ways. When we build any kind of secure system we have to assume that the firmware isn't actively malicious, in the same way that we have to assume that everything else under our stack isn't actively malicious.
But Secure Boot isn't about protecting us from the firmware. It never has been. It's about limiting the set of objects that your firmware will run. Now obviously if a sufficiently powerful actor has leaned on your firmware vendor then they may be able to run arbitrary code on your firmware, but why bother? They could just have the firmware include some SMM code that'd trigger in specific circumstances and modify arbitrary addresses in your running OS.
Obviously Secure Boot does nothing to protect you against such actors, but that doesn't mean it adds nothing to security. Microsoft have signed literally hundreds of binaries. Fedora have signed significantly fewer than that, and all the ones signed by Fedora have also been signed by Microsoft. Removing the Microsoft key and only trusting the Fedora one clearly improves security, if only because you'll no longer be able to boot the Ubuntu grub that'll happily boot unsigned kernels. Perhaps you weren't aware that Microsoft is effectively the global signing authority for UEFI binaries?