You are overlooking something very important.
One can readily "get familiar with firearms handling in general and different platforms", and most people can, with practice, shoot reasonably well with "almost any platform".
But in SD situations, a person must react extremely quickly, without knowing in advance, and without taking time to ascertain, which gun he is drawing in that instant. He must adapt to the "platform" in the instant, without fumbling and without delay.
Carrying different "platforms" in a "carry rotation" complicates the task. It's not like practicing at the range.
This is not untested theory. There is a science about it. Those who design weapon systems, vehicle equipment, displays, and just about everything else involving potential danger, subject their concepts to human factors engineering analysis.
A case in point: most people believe and will contend that they can drive their daily driver as well as their spouses car. Sure--but sometimes something unexpected happens in the middle of an intersection. The emergency becomes the telling point. This comes up in court cases.
The same thing can happen in a self defense incident. Self defense is not about "going shooting",
Some years ago, a top aircraft industry exec took over a major car company. He decided to drive every one of their models. The wipers, headlamp controls, fuel cover releases, and other things were annoyingly different. This led to high level direction at a staff meeting.
I studied human factors engineering and a lab course almost six decades ago. I could have told the folks in Dearborn something then.
Some people my age, and some old car buffs, may remember when the automatic transmission selector patterns differed among car models. There is a reason why P-R-N-D-L is required today.
