First, Bernie isn't in dire health condition. Bernie has a blockage, and had two stents 'installed'.
Doctors will say that stents are a simple procedure now, and you might walk out of the hospital on the same day of the procedure. Generally, most people then get this lecture by a doctor that they are either eating in a way that is not healthy, smoking, drinking too much, or living a lifestyle with too much stress. It doesn't mean the lecture is BS or worthless....it's just how doctors treat this.
The fact that Bernie is 78 years old? Well....here's the harsh reality. Bernie doesn't smoke, and I would assume that he's mostly a coffee or soda guy. So the suggestion here is that this presidential election stuff is triggering stress that he really doesn't need.
Him retire? Zero chance of that.
The stents simply telling you of a future problem with more chest pains? Maybe.
If he were to have a heart attack and resign or be unable to act as a Senator? The Governor does not have the authority to appoint someone. He has the authority to call an election....within 90 days of the seat being empty. The current Governor? A Republican.
The odds of this scenario playing out? In a normal political year, I'd say less than 10-percent chance. However, the last twelve months have been weird in the US, and it wouldn't shock me if suddenly you had Senator with a heart attack, and some race on to replace the guy.