Secure Boot provides no useful security for an individual user on the machine they own, and as such should be disabled by default.
If you want to enable it for enterprise/business situations, thats fine, but one should be clear about that. Otherwise you get the exact Microsoft situation you mentioned and also no one knows about it.
So everyday users should be vulnerable to bootkits and kernel-mode malware...why, exactly? That is useful security. The fact that people do not pursue this type of malware very frequently is an effect of SB proliferation. If it were not the default then these attacks would be more popular.
Citation please.
There are so many vectors for malware, can't say I'm just going to accept this one on pure "because it's possible."
You're arguing for not wearing seatbelts because no evidence has been shown that anyone has ever been saved by wearing one has been presented. That's just stupid by refuting ubiquitously understood data and facts.
SecureBoot ensures a valid, signed OS is installed and that the boot process generally hasn't been completely compromised in a difficult-to-mitigate manner. It provides a specific guarantee rather than universal security. Talking about "many vectors" has nothing to do with SecureBoot or boot-time malware.