Petru, while I agree with everything you say, 100%, I have to ask, "So what?"
The Creedmoore is being marketed as an across the course target cartridge. As such, it is highly specialized, and one would not be put off at any of the "drawbacks" that you outline. Most shooters I know that are putting $2,500+ into a target rifle aren't going to worry about setting the barrel back and rechambering in a couple of years. And most competitors I know are not hung up on a long action AR platform.
I don't see the Creedmoore eating into the 308 market, or any other market. OTOH, there is a good deal of knowledge that can come out of a new cartridge that is more efficient than than the 308 Win.
Now, I love the 260 Rem. Is it truly a "better" cartridge? In many respects, it is. But maybe not as an across the course target cartridge. Time will tell.
Oh, and I think your point about less tapered cases and dirty chambers is more of an indictment of direct gas impingement platforms than "improved" case design. Most turnbolt target rifles are rarely affected by dirty chambers.
One of the great things I love about America is the choices we have.