I suspect .357SIG will remain on the commercial market for some time. There's no shortage of folks who like to shoot their guns for the "wow" factor, meaning muzzle blast effect and noise.
In the LE field the market has been declining, though. I don't keep lists of such things, but I usually hear of more agencies transitioning away from .357, here and there, than to it.
When I asked someone at S&W why they dropped the M&P357 from their LE catalog, I was told that they'd still make a production run for any agency who wanted to order M&P357's, but demand had dropped low enough that it was no longer considered viable to keep it as a standard production model/caliber model.
When I did another Glock armorer recert earlier this year, I asked the instructor about seeing agencies using .357 models in the classes. He said he couldn't remember the last time he taught a class where an agency was carrying .357's, and it wasn't considered a caliber that saw a lot of LE use. He looked around and asked some other armorers in the class if they knew of anyone carrying it, and the only example anyone could think of was an agency who carried a different brand of duty weapon in .357. This was pretty much the same thing that's happened in previous Glock classes, too.
I actually know another instructor who owns a Glock in .357, and he effectively used it in a shooting some years ago, but he doesn't carry it anymore. He prefers to carry 9mm or .45 ACP.
I suspect that there may be a regional influence involved in some places, meaning where a large agency may carry it, so some smaller ones might follow suit.
It's not a caliber I've often seen on the shelves in the occasional stores where I've looked in CA, OR & WA in recent years.
I've still seen a lot more .357 Magnum than .357SIG on shelves, but it's not surprising that the .357 revolver caliber remains more popular on the commercial market, considering the huge numbers of revolvers that will remain around for many years (and the greater versatility of the Magnum revolver cartridge, over the pistol cartridge

).
But hey, there are still folks who enjoy owning and using 10mm and .41 Magnum, so why not .357SIG? That's one of the great things about America and the American firearms market, after all, right?
