Cartridges come and go. If the firearms industry didn't continue presenting new products (firearms AND ammo) to the gun buying public, they would probably go broke. The firearms industry needs to make the gun buying public perceive a "need" for the latest, greatest, whiz-bang, cartridge or firearm to keep them buying new products and accessories.
The 30-40 Krag is a very old cartridge, but it will kill deer just fine. While newer cartridges will shoot flatter, they won't make a deer that is properly hit any more dead than dead. Dead is like pregnant...they are, or are not.
For those that feel the need to argue over the effectiveness of cartridges at distance, the anti-tank rifles of WWI have plenty of power.. but my point is that the firearms industry needs to continue creating a perceived "need" for new products so the public continues to buy those products.
Various cartridges come and go in terms of popularity...that has always been the case. Some cartridges have a longer popularity than others. The 30-06, 45ACP, 9mm, 8mm Mauser, 7mm Mauser, 45-70 are among those cartridges that have proven themselves over a very long time. Newer cartridges such as the 6mm Creedmoor, 6.5 PRC are great, and while originally designed for a specific purpose, they are also very useful for many other purposes.
I don't get worked up when new cartridges come onto the market place, nor do I mourn when a cartridge's popularity wanes into relative obscurity.
If all ammo makers quit making ammo or components for the 260 Remington, I could easily form cases and make ammo somehow. It is the same for many other cartridges that slip into obscurity.