Much like the 10mm and the .41 Mag (both of which I own, for the record) the .357 sig will probably be around for a while.
But...there is a difference between "hanging around" and "thriving". I don't see any of those three thriving or growing significantly in popularity. The difference is that the 10mm and the .41 Mag (ammo and firearm sales) are supported strictly by hand loaders and enthusiasts.
The .357 Sig has various LEA use. When/if they move on to "the next big thing"...I'm not sure that there are a lot of enthusiasts. Some, yes, but I only know two guys who shoot .357 Sig. One is a hand loader, one has owned his for at least five years and I have seen him take it out once.
If you have a set of dies and some cases, you will be able to get projectiles, primers and powder, and you will be self-sufficient...just like we 10mm/.41 mag guys are.
Don't assume just because a law enforcement agency carry a gun for a living means they are experts.
Bingo. The government, at every level, has a long history of making decisions made on ego, kickback, or low bidder. The people who make the decisions are administrators, not field/street operators. This applies at every level from fire hydrants to F35s.