@Hunter-Habib
I think that all post ww2 calibers you listed were new in their performance and found very good place in hunting world for them, making them classic
By early 2000, hunting world had everything from .17 up to .700, in increments of 0.5 mm, and case capacity plus additional rimmed versions.
Then, enter 6.5 CM, when there already is 6.5x55, and 6.5x57.
Okey, so here it is. New baby is born. But nothing revolutionary.
Then you have "grumpy old men".
That would be older generation of seasoned hunters. Old men, sometimes hardly accept new things. But they also remember good old things.
That makes conservative part of market.
While in Europe, making of new caliber is exercise reserved for renowned factories and carefull planning and marketing strategy (30R blaser, or 8x55 blaser come to mind, recently), in America (much liberal in its gun laws, and tradition of home gunsmithing) invention of various wild catters is in dozens. But many new calibers dont live long enough, to make history.
But also, this means American market is flooded with caliber options, and factories always need something new to sell. USA market is quite saturated with calibers and guns, in last 100 years of good old 2/a.
In this kind of environment 6.5 CM is created.
When something new is needed.
CM means creedmoor, an old rifle range where first long range (1000 yards) matches were organised by the end of 19th century, giving a taste of extremely effective, and accurate long range cartrdidge, just by its name.
Marketing policy followed, gun writers followed, and new star is born, but in reality, nothing more then good old 6.5 Swedish.
But old generation of hunters and rifle owner have memory of elephant, and they know it is really nothing new. 6.5 CM came just when old 6.5s were becoming a bit forgotten, which is the first reason which created a space for marketing of new product.
6.5 CM, mainly accepted by younger generation (though not exclusively) who never used older 6.5s and old generation remains at least a bit more "scepetical" (though mot exclusively).
And new internet debate is born!
6.5 CM for who, and for what, is it better then old 6.5, etc. ?
Marketing is strong force, once when 6.5 CM hype caught up, followed also good ammunition supply on the market, with all kinds of bullets, from hunting to match and low drag, with high BC.
Once you have established a new caliber, with excellent ammunition market options, you create popularity. Today there is more 6.5 CM ammo available, then for 6.5x55, or 6.5x57.
But, mind: old 6.5x55 is Swedish "national" caliber to hunt moose (means effective). And it was a sniper caliber. (means accurate)
In todays metallic cartridge ammunitions', is hard to get something really new. But marketing ways can make it new.
I was considering for a while 6.5 CM, for target purpose. But on the end I decided to stick with 308 win, for the ranges that I shoot targets. (up to 600 meters)
And interestingly for longer distances, new wonder kid in open class is 284 winchester, 7mm class.
I see this cartridge, more as a marketing success, then a ballistic break through, but as far as I am concerned it is welcome to stay with us.
In short this would be my overview on social acceptance of 6.5 CM, in our small gun owners world.