If they run away while still holding the weapon in a ready position, you could reasonably argue that, and there were cases when people did, and convinced the jury.

And I think it's reasonable - why should you try to guess whether it's retreat or merely changing position? And even if it's retreat, it doesn't mean that they won't shoot back as they do so (indeed, "fighting retreat" is a military term). The whole point of firearms is that they can do harm at a considerable range, after all. If the attacker wants to make their intention to stop being a threat clear, they can drop the weapon. If they don't, I'd rather err on the side of not second-guessing the victim.