Erm, it's a good question, but the problem is that it DID kick in and that Alzheimer's is damn hard to butterfly away. Worse, we have no idea exactly when Alzheimer's disease kicked in properly and started to affect Reagan. Frankly I have massive suspicions that it happened earlier than his White House staff claimed. However, I have no proof. His son, Ron Reagan Jr, claims that he had suspicions in 1984. If true that's devastating.
Anyway, in the unlikely event that he'd avoided it and lived long enough to witness 9/11 and Dubya, it's likely that he'd wonder what the hell was happening to the GOP, as it seemed to be going to hell in a handbasket.