The story of cognitive decline was pushed because it was true. That's why he basically did no press, and why he used a teleprompter during times no other president would: https://archive.is/C60uH
When he did speak without one, he regularly said stuff like "they don't want me to talk about that."
I don't disagree that they're both way too old. However, the Trump we have now seems to be basically the same Trump we had in 2016. He rambles about as much as he did then. He could quickly deteriorate though - we'll see.