The good doctor is making that assumption. His time frame, as repeatedly stated, is on the order of a half-century. Fine, no problem. While Africa, say, may not change significantly in 50 or so years, or South America perhaps, Russia and China most assuredly WILL change, rather radically in my view, over that same time span.
I believe if you read Barnett carefully you will discover that he suggests that China and Russia have a lot of challenges to face.