Knowing the amount of actual control that the President wields over policy, as well as the number of staff, aides, and cabinet members around to keep things in line, I'd much rather a president who wasn't "all there" than one who attacks the democratic process, stirs up dissent, actively defies the legal system, is erratic in his dealings with foreign countries, and is generally just a horrible human being.
And, for the record, I think that right-wing media has overly exaggerated any cognitive difficulties that Biden may or may not face. He's certainly not as sharp as he was 20 years ago (and neither is Trump, for that matter), but I don't think he's the senile old man that Fox News claims he is either.