Improvements in bullet technology have made the .40 S&W less necessary, as much more-effective 9mm ammunition is now widely available, compared to when the .40 S&W was being developed, and introduced. I was a fan of the 10mm and 40 S&W, when they were introduced, but am now confident that I can defend myself with the better 9mm duty/defensive loads, available today. Nothing wrong with .40 S&W, but 9mm is “more right” than it used to be.
.40 S&W is more snappy than I want to fire, in a duty-sized pistol, these days. I am not so young, anymore. I carried a .44 Magnum duty revolver for a year, and then a .41 duty revolver for five years, in the Eighties. It is wonderful to be in one’s twenties, and immortal. Next, was three years carrying .45 ACP duty pistols, in the early Nineties, followed by four years of .357 Magnum duty revolvers, then back to .45 ACP for five years. (These transitions were voluntary.) Then, from 2002 to 2015, I was required to carry .40 S&W duty pistols. In 2002, .40 S&W recoil was piece-of-cake. By 2011, at age fifty, .40 Snap & Whip, when fired from a light-alloy-frame, high-bore-axis, somewhat-rounded-butt P229R duty pistol, was becoming problematic, for some of the joints in my right hand and wrist, whether I was shooting right-handed, or even two-handed lefty, with a “proper” thumb-forward support grip. (I do better with squared-profile grips.) So, it was a relief to transition to an “orthopedic” 9mm Glock G17 primary duty weapon, in October 2015, a month after my chief OK’ed 9mm duty pistols.
FWIW, .45 ACP remains fun to shoot, using all-steel, 5” 1911 pistols.