It's not a matter of what I might prefer or if I might be able to make a good argument for that preference, it's simply that the premise "high-cap magazines are needed" is not supported by the example given.
What would be needed to make a reasoned argument would be two, similar situations wherein the good guy lost in one while using low-cap magazines but prevailed in the other, using high-cap magazines. There would still be much left to assumption, too many variables for a real conclusion, but at least we could see the rationale for the argument.
In the scenario in the OP, we don't know what difference, if any, a firearm of ANY kind might have made. Low-cap, high-cap, 9mm, 45acp? He can we know?
IF there had been a firearm and IF it were in the hands of the right guy and IF he had been able to employ it successfully and IF the bad guys didn't give up or run at the first shot and IF they continued to fight after shots were fired and IF...
What does it prove?
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.