6.5CM and .260 don't shoot all the same bullets equally well. When you get into the really long-nose, heavier bullets developed for long-range shooting, the CM works just a little better. And since competition is all about stacking up tiny advantages, once the idea existed, the reality had to be created.
If the CM had been standardized first, there would be no reason for the .260 to exist. But with the history that's out there (short as it is) for the .260, there has been a body of load data developed that doesn't exist yet for the CM. So if I want to have a good choice of hunting bullets with lab-safe load data, and don't care about long range competition so much, the .260 is currently a better choice. IMO.
Load data is also what distinguishes both of these from the 6.5x55 in my mind. Plenty of people say you can load the 6.5 to higher pressures for modern rifles, but I've searched, and when it comes to lab-tested data from reputable companies, even the data indicated as being for modern rifles only still doesn't give the velocities of the SA newcomers. Until I become more of a gambler or buy a strain gauge setup, the newer cartridges seem to have an edge. All of them will kill stuff just fine, of course.
As for the next 20 years...
The past year and a half has seen the spark and flameout of something that could have been really cool for the AR platform. .375 Reaper is basically 300 Blackout done with a .308 case. A short-range hammer with no concerns about cost or availability of cases. Unfortunately, the guy who first tried to commercialize it was such a poor business person that the round is for all intents and purposes stillborn. From an outsider's perspective, it seems that the guy was far more interested in promoting how incredibly smart he was to have the idea and extracting every dollar he could get his hands on RIGHT NOW than to take the simple steps necessary to build a user base for a wildcat and develop some modest commercial legs. It will be a few years before anyone could expect to do anything with the concept without the guy trying to slap a lawsuit on them. Maybe he'll end up having to go through bankruptcy, and someone will get the right to develop the idea without unnecessary legal expenses. Currently, the people who were ready to go once there was something to actually buy are either trying to forget about it or changing lanes to .375 Socom.
