I have been a fan of 38 super and 357 mag for a long time, but both have limitations, not necisarily of the cartridge, but in support. There are very few new pistols made in 38 super that are not 1911 platform, and only a model or two that are. I love the 1911, but not everyone does, and with striker fired, DAO, or DA/SA being the predominant sidearm and ccw type today, the lack of choices in 38super is it's main hinderance.
357 mag has several modern REVOLVERS chambered in it, a couple rare or impractical autos, but with a capacity limited to 7 or 8 rounds at most, many people shy away in favor of an extra 6 to 10 rounds they probably won't need to use, and proficient use of a revolver is a skill in it's own right, firing a long DA trigger is something a lot of people can't get used to.
This brings us to the 357 sig, it does most of what 357 and 38 super do, but with greater capacity than 357mag, and more choices in new pistols than 38 super. I do like the caliber, and hope it stays around, and while it is not my absolute favorite, it is hard to beat in a compact auto.