Not sure I have noticed the upswing in .40 cal selling? But the whole calibre question is one big load of crap anyhow!
The two big reasons I have for owning pistols, main one, self defense, plus IDPA competition.
Take competition, the maximum round count is ten (for fair play to the poor unfortunate souls who live California) and other places, where 10 rounds is the max capacity allowed.
The accuracy of my 9mm rounds, phenomenal, contributed by my Glock19 4 gen; pistol, with an about 4 lb trigger weight. Six inch plates at 20m no problem.
In blue above, my 1&2 reasons just as important.
Self defense, pistol rounds are wimpy, at best, all of them. Accuracy (location) being the key, more accurate with less recoil, more rounds in a 9mm pistol!
Flooding a single attacker with rounds, gives you a better chance of surviving an attack.
Sorry to go off subject, I do tend to do that.