Biden can walk up a flight of stairs. The problem was he was trying to
show the world that he's still "spry", so he was bouncing right up the
steps until his feet got ahead of his brain and the feet stopped working.

If he just walked up the stairs like a healthy 78 year old then he'd be
fine. It might take a few minutes for him to make it up to the door, but
he'd get there eventually.