"Popularity" probably depends on where you're talking about to some degree.
I only know one agency in the counties around me where it's the issued round. I personally only know one cop who's used one in a shooting (but who doesn't work for the agency which issues it, though). While the cartridge (and gun) performed acceptably for his situation, he carries a couple of different guns/calibers as his normal off-duty weapons nowadays.
The .355 bullets used in the .357SIG load have reportedly been revised and refined since initial offerings, as has the way the bullets are crimped in the very short case neck, apparently
Just talking to different ammunition and firearms companies reps, it seems the .357SIG still occupies a pretty small segment of the LE firearms market (a distant 4th place, so to speak), although it may be picking up a little steam now and again. After all, S&W decided to offer their new M&P pistol chambered in it, and I've heard they've sold some to a small number of agencies who carry .357SIG weapons. (NMSP stayed with .357SIG but switched from Glock to S&W/M&P a little while ago.)
Having listened to some different factory instructors during armorer classes, it seems the .357SIG still offers the potential for a bit more wear & tear on guns than the .40 S&W. Oh well, it's always recommended that dedicated service weapons are periodically inspected and serviced, anyway, right? No big deal.
The 'wear & tear' on the shooter is still a subjective matter, and folks can decide such things for themselves.
The velocities of the loads offered by the major ammunition companies who make the most ammunition for LE/Gov agencies aren't exactly a mystery. It's not uncommon to see 100-150fps more velocity produced in the .357 SIG ( look at the RA9TA 127gr +P+ 1250fps v.the RA357SIGT125gr 1350fps, for example). Speer seems to list their load being a bit faster than a couple of the other big companies.
There's also the small ammo companies who offer hotter loads (but who are probably more popular among the non-LE/private shooter folks).
Handloaders? Who knows? I was an avid handloader as a young man, but I haven't been interested in getting back into it for many years.
Now, I've owned .357 Magnum revolvers since I was young. I carried them as issued weapons when I was a new cop and revolvers were still commonly carried, too. I've always liked the .357 Magnum as a nice, medium-bore, broad-spectrum type of useful handgun caliber. I wasn't necessarily as enamored with the 125gr JHP load as a lot of other folks, finding admirable qualities in the 140gr JHP loads (and later a 145gr load), and even heavier bullet weights, but the caliber allowed for a decent range of choice in the way of bullet weights when it came to choosing a load for a range of tasks. Versatile.
Something the .357SIG doesn't really offer. The .357 is sort of a 1-trick pony, and although it's not at all a bad trick, it's still not what I'd consider a caliber capable of the versatility of the .357 Magnum cartridge in a good revolver. So what, though? I commonly carry a 9mm compact or subcompact, and that has sort of a limited application range, too, right?
No big deal. It appears Sig was just looking to capture some of the market capitalizing on the performance of the middle weight revolver Magnum load, anyway (hence the selection of the name), and it can reasonably be said that they seem to have done that to some extent. Good for them.
Doesn't mean I'm going to own a pistol chambered in it ... but I certainly don't begrudge anyone else from enjoying it.
I just haven't seen the cartridge in many guns in many LE holsters out this way.
Matter of fact, I know of a fellow who used to work for the agency I initially alluded to who bought a compact version of his duty gun. The gun he bought was available in .357SIG, which would have matched his one-time duty weapon, but for some reason he chose to buy the model chambered in .40 S&W. Go figure.
I think the .357SIG will do something the 10mm was unable to do.
I think it will remain in LE service, if only as a 4th place choice when it comes to total numbers of guns sold and ammunition produced for LE/Gov usage.
I suspect it might remain more popular among the private owner (non-LE) shooters ... and there's nothing wrong with that.
BTW, personally, I'd be very skeptical of labeling anything produced in a handgun as an "unbelievable manstopper". It's still just a handgun.
Just my thoughts.