Since I just acquired a .17 Mach II, I have run across several threads on forums that think that the .17 Mach II is a dead round. I have also heard this for years about the .41 magnum, but S&W reintroduced the model 58, but with the stupid lock in the side plate. I have shot a .221 Fireball for many years and remember about 10 years ago there was a scare that Remington was going to discontinue making brass for it because of it's lack of popularity.
I think the .45 GAP is the best candidate to disappear soon. There may be some limited interest, but it was conceived as a defense round and for that purpose will have a hard time surviving. The .17 Mach II and the .221 Fireball are good varmint rounds and fit a nitch, the .41 magnum is a good hunting round. The 45 GAP is what? Not a competition round, not a hunting round, not plinking round, just a very narrow following for self defense. Some other cartridges survive because they are reloaded for a particular purpose, like the .22 Hornet, but hand loading a .45 GAP for self defense?