Well, if the attacker has a gun drawn on me then there is nothing I can do.
I've seen videos of people successfully drawing while a gun is pointed at them. It's not a great strategy and will probably get you shot, but it can be successful.
And if they took their focus off me, I can't legally attack them.
You can't ever legally attack them, you can legally defend yourself against them if they pose an imminent deadly threat. Taking their focus off you doesn't necessarily mean they are no longer a threat. If they are taking their focus off you to leave because the crime has been completed, then shooting them would be legally problematic. If they have taken their focus off you momentarily but a reasonable person would believe that's only temporary and the threat is ongoing then shooting them would likely be justified.
They did not kill me so my life is not endangered.
Incorrect. The fact that an attacker has spared you so far isn't proof that you aren't in deadly danger. It only means you haven't been killed so far.
Only if I know for certain that the attacker is going to kill me.
No, you don't have to "know for certain". You only have to have
reasonable belief that your life is in danger. Even if it turns out later that you weren't actually in danger (e.g the attacker's gun was actually a toy) if a reasonable person in your position would have believed they were in deadly danger, then deadly force would be justified.
Just to clarify, I'm not saying that it's never justified to shoot someone in the back of the head, I'm just saying that it wouldn't be my Plan A. It's not all that often that a defender will have the opportunity to shoot an attacker in the back of the head.
It seems the question is which gun would I rather die with my gun that I like the most or a Saturday night special?
This is a double strawman.
1. No one is suggesting that anyone carry a "Saturday Night Special", just that the choice of what gun to carry should probably be made on the basis of suitability for the desired task vs on the basis of which one is the favorite gun.
2. The deadly force laws are somewhat restrictive, but they do provide protection for a person who has no reasonable alternative but to shoot in self-defense. Too many people focus on the restrictions rather than on the protections they provide.
It would be just as simple - and just as unrealistic - to design a problem which cannot be adequately met by a modern high-cap auto. And it would be just as foolish to then hold up that scenario as evidence that a modern high-cap auto is a poor choice for personal defense.
It's always easy to create unwinnable situations. That doesn't mean that all self defense tools are useless and it certainly doesn't mean that the choice of tool makes no difference at all.