It's actually very simple and makes perfect sense. If McConnell dies before August 3rd, a special election to fill the seat must be held. It's widely believed that Thomas Massee, a staunch Trump critic, will win it, and it will shift the balance of power. If McConnell is allowed to lie in a vegetative state until August 4th and then dies, the seat will remain vacant until the midterm elections, and the winner won't be seated until January. Republicans believe that it's better to have an empty seat for the rest of the year as opposed to one filled by someone who will align with Democrats.