As far as "handguns" go, Ive probably got more .45acp's downrange than anything else, although 9mm is catching up pretty quick these days (and actually may well have surpassed it).
Having owned a few 9mm SMG's over the last 30 years or so, and having access to a bunch more, 9mm is by far the winner in the numbers count. That count is probably more than many people have ever seen, let alone shot.
I quit using .45 in my carry guns about 10 years or so ago. Havent had any desire to go back either. I actually sold off most of my .45s, although I have kept, and still shoot a few. Had a brief fling with 357SIG, and after awhile, realized it really was nothing more than +P+ 9mm, so now its 9mm riding in holsters for the most part these days.
With ammo advances, the gaps have narrowed for the most part with the major players. Take that out of the equation, add double or more the ammo capacity, in a gun thats usually smaller, and usually a lot easier to shoot faster without loss of accuracy with, whats not to like?