Citation for what? The existence of bootkits?

Petya/NotPetya, Alureon, Carberp/Rovnix, Gapz, LoJax (firmware rootkit!).

All of these attacks would be thwarted by SB (and in Petya's case, simply having UEFI enabled at all, since that was only for BIOS machines)

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.