The 6mm Remington is a better cartridge than the .243 Win, but I live with the .243. The .280 Rem is better for larger animals than the .270 Win, but I live with the .270.
We don't really need to have more cartridges, we should be happy that we have great cartridges still being made. The big box stores are dictating which rounds will continue, just by offering them on their shelves, whether you or I buy our ammo there or not. Try finding .260 Rem ammo at Wal-Mart, even 7mm-08. You might find a box or two, but it probably won't be YOUR favorite brand or bullet weight.
Manufacturers look at sales figures and delete those cartridges that don't sell well. Sales, not specific qualities of a particular cartridge drives obsolescence. Believe it!
(I'm not talking about the recent aberration cause by recent massacres and following political threats to firearm/magazine ownership.)