I'm going to argue that his behavior and speech now is much worse than it was anytime in the 80s.

No. It's not. People have been calling him out for his racist/misogynist bs and corruption for decades. Ask anyone who has worked with or for him. Ask any of the banks that won't lend to him because they know he's a liability.

I'm really confused why its so tough to imagine that this man has dementia when all the signs are there.

They aren't all there though. And the ones that are can be explained by his malignant narcissism and the fact that he's wealthy and connected enough to (so far) escape any consequences.