Trump appears to be suffering from a serious cognitive decline since he was last president. He was never the sharpest spoon in the drawer, but it's bad now. If he's still alive in five years, I don't think he'll be capable of doing anything other than drooling on himself.
Trump is old. Republicans have been stressing how old Biden is--and using his life-long speech impediment as evidence--but Trump trails Biden by only four years of age. They're both at an age where you start seeing a sharp divergence in cognitive abilities, where some people take a nosedive into dementia and senility, while other people retain their mental faculties while their body fails. Trump seems to be the former.