Declining with age is not the same thing as being incompetent. Not even close.
Biden no longer had his fastball. However, if Biden was throwing underhanded, he was still far more competent than deranged, evil Trump.
Presidential campaigns have nothing to do with whether or not someone would be a competent president. Trump is living proof of that.
Trump was able to win 2 presidential elections. But that does not equate to being a competent president. In fact, he has proven repeatedly that he is not competent enough to manage an Arby's, let alone a country.