The AI thing is cartridge dependent for a lot of reasons. Not sure what the case differences are from .280 and .260 (other than neck dia). Or what the AI dimension change is. Usually it means less taper and the 40deg shoulder, which means more case capacity and longer case life. Some cases are already 'overbore' a bit and the extra case volume results in little gain. The last may be the reason the 260AI is not as popular compared to the 280AI?

I think it depends more on what people do with it. Do they expect to make a sluggish cartridge a barn burner? Or maybe the factory twist won't let them 'use' the extra case capacity to it's full potential?

The CM is one of those interesting ones. Designed for short actions and then hyped up. But, it remains as a pretty efficient cartridge, which usually means better accuracy. Yes, the 260 and 6.5x55 work well for higher velocity and many accurate rifles were produced for them. I was considering a 6CM when I was looking for a 6mm cartridge. It is not too overbore to ruin barrel life, but, it has enough capacity to go over 3000fps with the heavy bullets. Interesting enough, I went the other way with a plain 6BR. The extra velocity made no difference for my use and the extra long barrel life appealed to me.

I do agree that many have a 6.5CM for very wrong reasons, expecting it to take down elk or moose with 'magical' performance. OTOH, there are those in the accuracy game who like the niche the 6.5CM fits in. And whatever you or I think about the cartridge, the various rifle manufacturers had stepped up with premium rifles in that caliber. Several folks at our long range shooting site like their 6.5CM's for the 1000yd performance out of factory rifles. For some it was the first time they moved from the .308 to something else.