Maybe derailing a bit, but why the 9mm (or .40 S&W).
Flatter trajectories have nothing to do caliber selection (as you noted). SOF handguns are carried as a backup to a long gun, which is used for 98% of engagements.
9mm usage is near universal in SOF because the military supply system provides plentiful amounts of it and most of today's issue weapons are 9mm [M9, M11 (SIG 228), SIG 226, , Glock 17/19]. .45 ACP is less available these days, which limits the amount of training that can be conducted. It's easier to get .45 when already downrange and less so when trying to build an annual home-station range training plan using annual STRAC allocations.
Another unit that formerly used custom 1911s went to Glock 22s a few years ago. A lot of old timers in that unit were resistant to the idea, but several years of practical use with the weapon & caliber in combat apparently proved its utility. For the most part, they like the weapon, the comparable terminal results, the increased magazine capacity, and the enhanced barrier penetration (compared to .45 ACP).
Some Army SF ODAs elect to go with .45 and have every man carry a 1911 in lieu of the already issued M9. But it's an "All or None" common choice. The guys are rarely afforded the opportunity to individually pick and choose what they carry. Specialized versions of a common issued platform...yes. Personally owned exotic bullet launchers...no. Shooting drills, personal kit, and TTPs are organized around a training framework based upon commonality of weaponry, accessories, optics, and ammunition.
DoD still maintains sizable stocks of 1911A1s available for use by selected units...usually drawn from depots and then customized in-house as required.
I've never seen anyone in SF using a blowgun or a slingshot (except as an individual screwing around with a personally owned item), but I know a legendary SF Team Sergeant who actually shot flaming arrows (from a personal hunting bow) against insurgents in Iraq during a firefight. Both sides were utterly amazed.
Generally, .45 use today is by exception with about the same amount of folks using .45 as use .40...and everyone else using 9mm (about 95% of US SOF).
All three calibers will do the job and are accurate enough. It's not that big of a deal. Pistols are marginal stoppers in almost any duty caliber. Shot placement (skill) is the deciding factor with combat handguns. Followed by penetration. Both of those distantly followed by caliber, bullet, & platform. Naturally, expanding bullets
tend to work better than FMJ. My $.02