In the last what, fifteen years, we have seen a plethora of "new" cartridges introduced by Winchester, Remington, Ruger, and Weatherby.
Every single Remington short magnum is already obsolete for all intents. There were a very few rifles chambered to the Remington offerings by other manufacturers early on, but that practice is now ancient history.
The Ruger magnum class (short and long) offerings are less than a few years old in most cases. If you look at their current catalog, a great many of their short offerings are already discontinued. That should say something about sales graphs.
The Weatherby offerings are in the "super" magnum class, and clearly do not impact day to day Wal-Mart sales in any respect. These are boutique rifles, almost proprietary, and as such are some what outside the realm of the subject.
But the Winchester Short Mags, the .270, 300, and to a lesser extent, the 7mm cartridges still have an active market share plus these same offerings are chambered by a great many other manufacturers. Even Ruger still chambers the .300 WSM but has dropped their own .300 from current production on many models!
I rather doubt those three are going to be cut short any time soon, but if you are that doubtful of their continued existance, but yourself 500 pieces of brass and a set of dies. Problem solved!