In each of the historical examples you provide, the president of that time is the lone person in the executive branch experiencing the injury or health condition leading to his death or debility. In the current situation, there are numerous individuals in the executive branch experiencing the same health condition that our president is. More concerning than his health are the behaviors that opened the doors of the West Wing to a deadly virus. Regardless of how the markets react, this is a national security nightmare. One that our worst enemies could have only dreamed of.