"you should have run in the first place..."
Good thought, assuming running is an option.
Of course, one could be in an enclosed space, with the door on the wrong side of the problem.
One could be under attack by dogs; most folks can't outrun those, and hitting them can be a trick, as they tend to use speed and motion in attack mode, so multiple bullets per dog is likely needed.
One could be injured, or older, or obese, and unable to run under any circumstances.
One could be dealing with an assailant who is hyped up on any number of chemicals which would minimize pain and shock, requiring a lot of good hits to stop the problem.
I've never heard a convincing argument why a greater magazine capacity is a liability.
Now, going back to the "armed forces" aspect of this thread...
Napoleon - "An army marches on its stomach."
Von Clausewitz - "Amateurs study tactics; professionals study logistics."
If you can't carry enough of what you need, or get enough of what you need, then your needs are a problem.
Spending a lot of money to create stockpiles of ammo for a secondary or tertiary weapon is probably not strategically sound practice.
Ergo, it makes the most sense to use commonly available, relatively inexpensive, highly portable ammunition - which brings us right back to the 9mm for the foreseeable future, at least for the mainstream military. SPEC OPS folks have their own budget, and their own needs, and are really a separate discussion.