While I don't disagree at all; it is not a serious consideration for most hunters/shooters.
You have to get way out there before the advantages of a 6.5CM begin to be real factors.
If I was target shooting at long ranges and wanted (or was required to use) a short action, the 6.5CM may very well be the best choice. Not sure.
The 6.5CM gets a lot of hype and a lot of marketing. I'm not saying its a bad cartridge, I'm saying I've seen this act before.
Short/fat cartridges are very efficient and can be very good performers. But there needs to be big gains before before those improvements really tip the scales.
I'm not disparaging the 6.5CM but I'm not convinced the gap between the 243 and 7mm-08 is big enough to require some short action cartridge in the middle. The marketing people think I'm wrong and I may be.
SO - getting back to the OP's question about the 243 being in danger because of the 6.5CM in the market - I just don't think there's a big enough gap between what the 243 does and what the 7mm-08 does (both outstanding cartridges in my book) that the 6.5CM will kill off the 243.