"It is all but impossible for one to count the shots fired under the stress induced by a defense situation and/or attack."
I can't argue that emphatically about this, just throwing another into the realm of possibility. It would be easier to drop a mag with "some" left in it, and also the "mass shooter" may be (in his twisted mind) in one of the lower-stress gun situations there is - premeditated with static targets.
The shotgun with its "shoot once-or-twice", "load one-or-two" sequence may also be effective in this situation were psychos to make more use of it.
All else being equal, of course a reduction in the mag capacity of a gun will reduce its operational utility for a wide set of uses. We're now seeing how this is a very useful attack vector for incremental gun bans - in some states now the "10" is becoming 7, next stop 5?
One of the tools in this wave of gun control (on both sides) is misdirection, trying to make the other side not care, or focus on something else. I threw in one more for that list; but I acknowledge it's a marginal contribution to what is probably a marginal list.