It's an interesting point, but surely it is useful to know when you have been compromised (e.g. to change passwords)?What about post-infection where you want to e.g. clean up the machine without reinstalling the OS unnecessarily? Or in general preventing rootkits from persisting across reboots will at least help to prevent new data being exposed?

The only way to actually protect user data is to chain trust off the protected bootloader and kernel. Secureboot is necessary, but not sufficient.