40's suck because when semi auto's became the rule, most people bought into either 20 round 9mm's or 7 rd 45's. Stopping power data clearly showed 45 superior.
Then the FBI made the 40. It's stopping power data was collected and was on par with 45's, but in a 9mm size gun and it was easy to shoot.
Since the shooting culture was already split 9mm, 45 or 357, new shooters were the 40 market. People like me. We bought our first handguns as 40's. being new, we sucked, so 40 was called inaccurate or whippy. As we got better, it became a solid competition caliber...major + high cap.
Then when CCW became huge, 40's were made in 9mm size guns which 45's couldn't be made in.
Last, some of these newb's became mouthy to one or both of the old guard. The Glock tried to kill 40, by making unsupported chamber 40's and putting them in LEO and civi hands...and don't forget the kabooms.
So, in the end, the 9, 40 & 45 are here to stay.