I wouldn't say his cognitive skills are in a major decline. Far from it. He has withstood 2 attempts on his life and recently some serious threats. He has also stood up well to the rigors of a demanding campaign.
At 84, my friends remark on my mental agility, so by the signs of it I would say Trump is doing fine both mentally and physically.
My favorable comments have nothing to do about whether I like the man or not. It just happens to be the truth.