The "login GUI" is actually the process that launches user's sessions and then passes control of the screen/input to that session. It has to have "better than"

The "login GUI" is actually the process that launches user's sessions and then passes control of the screen/input to that session. It has to have "better than" administrator level access to do so (since it has to broker administrator sessions). Even if you ran the actual GUI as some special user, that GUI would still have to be able to do a bunch of powerful things using SYSTEM level services, so any exploit would be equally as powerful if it went after the SYSTEM login GUI or the login service. What you're suggesting would be meaningful feel good security with no actual teeth. Attackers would just use the boot disk to alter a different file or process.