Good post, Tim.
There are, no doubt, a bunch of sheep who will follow wherever they think the crowd is moving - toward .40S&W, toward 9mm, toward .45auto, whatever. At present, the trend of the crowd is toward 9mm. That said, there are plenty of people who continue to prefer .40S&W or .45auto over 9mm, but they're not getting the "press" like the 9mm flockers are getting.
.40S&W is a great defense caliber which, as you discuss, offers very near the performance of .45auto along with nearly the capacity of 9mm. And with well-chosen pistols, does so with ease of shooting and rapid follow-up shots. A lot of us understand this. But there are plenty of folks out there who don't yet understand this. Some will pay attention to this and benefit from it. Some will just follow wherever the apparent populist movement leads them.
I'm not implying that choosing 9mm is bad. I've got plenty of 9mm pistols myself. They're ok choices. And 9mm is especially suitable in subcompact pistols. But most of the time, I can easily carry and conceal mid-size and full-size pistols - and in such platforms, .40S&W really shines in performance, capacity, and ease of handling.
Remember too that a block of ballistic jello gives a uniform media to compare different bullet performance, but it is no-where near representative of the human body. And that in a defensive scenario, where an assailant's arms are likely held out between his body and yours, your bullets will likely need to bust through hand, wrist, forearm, upper arm bones, several layers of clothing, then sternum or rib cage, then continue on without being deflected to penetrate deeply and, hopefully, expand. Heavier weight bullets tend to bust through barriers and penetrate more deeply (bullet design held constant) without deflecting or fragmenting than lighter weight bullets. They have more momentum. This factor is NOT represented in ballistic jello tests.
While penetration and expansion measurements can be somewhat close between some .45, some .40, and some 9mm loads in the consistent media of gelatin, there clearly are differences in these performances. And there are differences in how these different bullets penetrate barriers such as glass, wallboard, human bones, etc. - differences which are not reflected in ballistic jello samples.
One can minimize these differences if one chooses to. Or one can recognize the differences and the potential to have to bust through hard bone or other barriers.
All things considered, if I have the choice between the best loadings in .45auto, .40S&W, or 9mm, my choice is for the best loading in .40S&W because it does demonstrate a better performance than 9mm does, closer to that of .45auto, and the capacity remains very close to 9mm. Others may choose .45auto or 9mm for reasons that make sense to them.