He's the only person in the country who can pull out a 9mm and shoot Trump in broad daylight without it being a crime. As well as every SCJ that voted for this immunity. He can then replace every SCJ he executed with his own appointments. If Congress won't approve his appointments, he can keep killing them until the vote passes.
This ruling is a free pass for dictators. All Biden has to do is act like one. I doubt he would, but he legally could.