A cartridge case is a powder container. Its design (shape [to insure incompatibility with similar cartridges and prevent mis-chambering (P.O. Ackley's designs not withstanding) and affect powder burn characteristics], taper (chambering and extraction characteristics), case wall thickness (ability to handle pressure), case head size (e.g. 6.5x55 has a different case head size than 308, 6.5 CM, 30-06, 8x57 and their derivatives which all use the same case head size), case head design (pressure again--think of the old balloon head 45 Colt cases), and etc , all play a role. Some are more aesthetic or are used to maintain a proprietary advantage (or both) like the venturi shoulder on the Weatherby cases.
When the design incorporates the ability to adapt to the evolution of technology, it provides a cartridge with staying power. I think that this accounts for much of the success in the real (not gun magazine journalism) world of the 6.5 CM. The fact that it is of modest recoil and can provide good accuracy doesn't hurt. The same can be said for the 17 HMR. I am still waiting for the 17 WSM to catch on (using the case from a power driver is a neat idea) but it is probably too close to the 17 Hornet and that niche is pretty small.
However, the introduction of new cartridges in order to increase market segmentation and generate more sales is a well-known and understood marketing tactic and the manufacturer's MBAs would be fools not to use it. They just need a population of willing buyers.
While this board has a number of highly-knowledgeable professionals (and we all benefit from their graciousness in sharing information), for most of us shooting, collecting, reloading, and hunting are all hobbies. And while some aspects are deadly serious and necessary to our survival (self-defense and subsistence hunting), it is the hobby aspect that drives a most of the sales and revenue for the industry.
One benefit of age is that you don't tend to fall for every new toy that comes along. But there are always exceptions. For example, it is difficult to find a 250 Savage, and it is still handicapped by pre-WWII design considerations. It is also hard to find a 257 Roberts (and many owners are highly reluctant to part with theirs and for good reason). But if a 257 CM were to be commercially introduced, it would be tempting to fill an empty 25 caliber short action slot in my collection. Not because I need it or it would do anything that my 6 mm Remington, 243, 6.5 CM, 6.5x55, or 25-06 cannot already do. But it would be sort of a neat thing to have...
Let's face, at some point it becomes more than a hobby: It becomes a disease.
