The question of why no “might kill us” as a class is simple. There is such a class, but if it lost the fight to kill us, it obviously was not ASI (picking a fight with the world and losing is pretty dumb), or it might win, at which case it won, we die. And then we will be in the same scenario for every AI stronger than it, and for AI weaker than it that might yet get lucky, just as we might get lucky and win at bad odds. The next AI we make will also want to fight us for the same reasons, and we will need to either fight it to (including preemptively, e.g. turning it off because a dumber model did something), or get a reason to believe that we will never fight it. And if you know you will fight your AI eventually, and you will win now, fight now.
The question of why no “might kill us” as a class is simple. There is such a class, but if it lost the fight to kill us, it obviously was not ASI (picking a fight with the world and losing is pretty dumb), or it might win, at which case it won, we die. And then we will be in the same scenario for every AI stronger than it, and for AI weaker than it that might yet get lucky, just as we might get lucky and win at bad odds. The next AI we make will also want to fight us for the same reasons, and we will need to either fight it to (including preemptively, e.g. turning it off because a dumber model did something), or get a reason to believe that we will never fight it. And if you know you will fight your AI eventually, and you will win now, fight now.