I have heard that one of the problems with the .243 v/s the 6mm Remington was the throating in the .243 was different than the 6mm Rem. The .243 throating was supposedly changed sometime down the road after that to give it better barrel life. Weather this really had anything to do with the problem of throat wash out I cannot say for sure.
As far as the 6.5 being around for another 20 years it will for sure be around in Europe as it has always been a very popular bullet diameter. The Swede's regularly shoot Moose with it.
I once shot some factory Swedish ammo out of my 96 military Swede gun and it was loaded so hot I was afraid the gun was going to come apart but the Swedes use this stuff all the time. I guess this dispels the myth that small ring Mauser actions will not take high pressure loads.
The Europeans have also used and still use other 6.5 calibers besides the 6.5x55. One of the very good ones is the 6.5x57 which is almost unheard of here in the U.S. The rimmed 6.5 Mannlicher and the rimless 6.5 Mannlicher are also used.
If your interested in a great read in the year 1900 Agnes Herbert and her cousin hunted Africa, and Russia all alone with no men and also Alaska (with their boyfriends in Alaska) using the 6.5 rimmed Mannlicher. She wrote three books one for each continent she hunted on. The description of the life of the indigenous people she encountered is worth the read alone not to mention the hair raising animal charges that almost killed her in Russia (a bear) and the animals that almost got her in Africa. In those days of the 1900's two young beautiful women traveling alone and of all things hunting alone was almost unheard of. To say they raised a few eyebrows in their day is an understatement.
There is one true story in the Alaskan book about Marital infidelity that is so hilarious it made me almost fall off my chair with laughter. My how little people have changed in 100 years. The story could have been written for a soap opera today.
The Alaskan book written by Herbert is quite easy to find but the other two are very rare. I found it very interesting when she mentioned how many Russians were living in Alaska in this time period.