I'm a new convert to the .357 Sig and I really don't see it dying off. There are a lot of LE agencies (USSS, many state police including TX, VA, NM, NC, DE etc., many local agencies such as Dallas PD and so on) that issue or authorize it. 45 GAP is a different can of worms, yes, it does have LE users and a group of civilian users. The problem with it is a)It is intended to fix a problem that really isn't a problem b) If you want a gun that fires it, you basically have to buy a Glock (unless you're one of the 8 people in the world who own an XD chambered for it) and it is almost impossible to find the ammo in stores, regardless of the ammo market supply/demand.
45 GAP is intended to be a flatter flying big bore round that fits in a medium size frame, 357 Sig is intended to closely replicate the performance of 357 Magnum fired from a full size revolver in a semi auto friendly size cartridge. Although .45 ACP is well known for being a great man stopper, it also has been shown to have questionable performance against hard barriers like sheet metal and laminated glass, 357 Sig has shown great performance against the same barriers. As long as Glock keeps making the 37,38 and 39, there will be somebody making ammo for them, but not many. I can count on two fingers the number of times I have seen GAP ammo on the shelf in a store.
The underlying reason for the existence I think is really driven by one intangible: Gaston's ego...
On another note, it annoys me to no end hearing/reading people say that the 357 Sig is a .40 S&W necked down to 9mm...THAT IS NOT TRUE!!! AHHHH!!!! :aargh4:
They were developed during the same general time period, .40 came out in late '91, 357 Sig in mid '92 and went into service with Texas DPS about a year later.