from what i have read i belive that the 6.5 grendel is a better cartrige however they both have there benifits. the 6.5 is better for long rang target shooting and has a higher BC. and the 6.8 seems to be better combat round for under 300M
also i found it very interestiiong that the 6.5 is still supersonic all they way out to 1100 yards now i dont have how long the 6.8 will
now to accutally answer your question i dont think either of these will go the way of the 45 gap because these accutally solve a problem unlike the 45 gap.
these bullets will remain usefull untill something else comes along that will do it better. i would guess that their popularety will increase as years go by
In my mind the 6.5, while ballistically better suited for long range sniping and hunting, is iffy. Only two manufacturers make uppers that i know of and for the log range stuff 308 is already established.
The 6.8 has been tried before and is actually a viable, practical option. Everything I've read seems to indicate it will last. The question is will it flourish or be a SPC? Maybe it's all in the name. I would go with the 6.8 SPC.
I believe that in both cases, if you reload, you would be ok since you could make your own brass if need be. In the case of 6.8, I would say that Remington will keep it alive, and I am sure that the 6.5 will be around a while as well. I know that up here the 6.5 sure is popular. They both use readily available bullets, no problem there.