Only a neurologist can officially diagnose someone with dementia. Everyone else's, including Congress' diagnosis is simply an opinion.
Ok, then why isn't he running?
Or look at it from an Occam's Razor point of view - for years videos have showed instance of what appeared to be cognitive decline - slurs, forgotten words, jumbled sentences, looks lost on stage, forgetfulness, walking aimlessly, stumbling etc.....too many examples to list.
Any time his mental acuity was questioned, he was not only defended by the admin and the media, but those questioning were frequently attacked. It appeared to get even worse at the G7.
Then the debate, which all but verified what many were thinking.
Given all that - why do you think he backed out? Would you bet your house against cognitive decline?