To me, mortal combat is any engagement where there's lives at stake.
Certainly, war is not the same as a civilian robbery, but when the outcome involves grievous bodily injury or death, to me, that's combat.
As for shooting until slide-lock:
For me, that's, with my 99% carry pistol, anywhere between 14 rounds and 19 rounds. That's a lot of rounds to, just like you've said, account for: and as good citizens, we know and understand the gravity of the potential lethality of each round - we are taught, and understand in our core, that we "own" each of the bullets departing the muzzle of our firearm. For many of us civilians, this is one of the overriding concerns for which we train. I agree with that sentiment, 100%, and I am always seeking to better my fundamental pistolcraft, based on this very serious and very real concern.
Do I expect that I'll ever burn through an entire magazine?
I honestly don't think so.
But at the same time, the most recent "good-shoot" CCW shooting in a Detroit barbershop goes to show that yes, in some instances, real-life does really mirror the Force-on-Force that we undertake, and that before you know it, you're at-slide-lock.
I shoot until the threat is no longer a threat. If that takes one round, all the better. Two? Three? Five? Eight? Eighteen?
Like Rob Pincus said, I'm not looking to always shoot X number of rounds at Y target: I'm going to be assessing the threat, and I will shoot to stop/neutralize.
If that's before slide-lock, all the better (but you'd bet that I'll have done a tactical reload, even if that only means that I've topped off the gun to lay down on the concrete, when the cops arrive). But if it goes to slide-lock from either a malfunction or from ammo expenditure, I know that I know how to deal with it.