Glenn, that is correct, but nobody is suggesting that we expect people to operate as a trained unit. Heck, even as armed folks, the majority of the gun community cannot even respond as trained individuals with their firearms, in part because of their lack of training. So no, you can't expect a group of people to respond as a trained unit or in any sort of coordinated fashion, yet there have been numerous instances of multiple folks responding simultaneously where such sequential or en masse hits have resulted in a cessation of firing. In other cases, it has only taken a single individual to effect a tackle that resulted in the cessation of firing.
Numerous active shooters do not pick fatal funnels. In fact, many don't seem to have much of a plan beyond shooting people. Many are out in the open or in a confined space for which they cannot control their surroundings.
The best time for the physical defense is BEFORE the first shot. Before the shooter works up to actually shooting there's some indecision and possibly some confusion that can allow a successful physical attack.
The problem is, almost nobody ever spots the shooter before the shooting and are in a position to do anything about it in the time frame before the shooting starts.