Well, while I often think to shy away from nibbling on this caliber bait ... I'll offer this, though ...
Whenever one of our folks used to come to me with this question (in my role as firearms instructor), I often pointed out a couple of things to them.
Firstly, I've carried both issued and personally-owned pistols chambered in 9, .40 & .45 ACP, and I couldn't have carried any of them if I didn't think they were more than adequate for the intended tasks.
Secondly, I invited them to try the different caliber they were considering, taking them downrange and letting them try a choice or two of whatever "caliber" they were thinking about switching to for duty or off-duty usage.
That little bit of self demonstration more often than not opened their eyes to things they might not have been considering, like whether a heavier recoiling caliber might work for them? Or, if they were still interested in going to a heavier caliber, were they interested in working to increase their skills to accurately and effectively use it?
Running someone through some normal drills (for familiarization), and then perhaps some faster paced and more demanding drills (to let them self-assess their ability to run the new caliber/gun against their existing one), better let them decide if changing calibers was a decision they really wanted to make.
Not everyone wants to put in the work that may be needed to run a harder recoiling gun, especially at speed and in demanding conditions.
As a matter of fact, the last couple of times we've allowed people to "pre-select" their new duty weapons (meaning caliber), we've seen a lot of folks express a desire to carry a .45 (last estimate I was told was the .45 was selected close to 70% of the time). Then, of course, once the guns were actually being fired for transition & quals courses, people were wanting to change over to lesser recoiling calibers (like 9's).
Don't get me wrong. I'm a long time owner and shooter of .45 ACP. I've always liked the low-power, but big-bore ACP round.
I've carried it as issued and personally-owned off-duty choices.
It's just that I've also found the 9mm and the .40 S&W to be fine for my use in the same circumstances.
For me ... learning to run and master the .40 made it much easier to run my 9's and .45's. The better I became with my .40's (issued and personally-owned), the better I shot my 9's & .45's
Before I started to acclimate myself to the .40, I found it easier to shoot my 9's after spending time shooting my .45's ... but range time using my 9's didn't really seem to help me improve shooting my .45's.
Nowadays, shooting my .40's a lot seems to make me a better shooter of both 9 & .45 ... and I've invested so much time shooting my .40's that I often have trouble distinguishing their felt recoil and muzzle whip from shooting my 9's. I couldn't say that 10 years ago. Back then the .40's felt like they had more snap & muzzle whip. (Well, having access to an agency's ammunition inventory, as an instructor, made everything a lot easier.

)
My .45's? Just a nice controllable push/shove and slower muzzle whip.
I tell people to suit themselves ... but remember that the "proof" of the appropriateness of their choice is still often found downrange, using a timer and considering the arrangement of holes in the intended threat targets.
Choose wisely ...
for yourself. If you're willing to fool yourself about your ability to controllably and effectively run any particular caliber? You won't have to look far to find someone to blame.
