Biden is indeed in a frail state now, but looks like he's accompanied by competent people at least. He's far from a polished president and statesman at this stage but he's still functioning and can carry out his duties alongside his team, so he poses no danger as being the sitting president of the US. Trump OTOH, as much as I enjoyed the clown back in the day, but with current turmoil around the world, Ukraine, Israel etc. he simply can't be trusted to keep cool and make the right decisions.