If you've watched OKC through the years, the duo of Durant and Westbrook wasn't going to get it done. Westbrook may be good, but his decision making and maturity make him a danger to be the guy with the ball in his hand every trip down the floor.
And yes, peaked. They were in the finals a few years ago. They've been to the conference championship several times recently. Had they won, they still weren't going to beat Cleveland.
Most blown leads in the 4th quarter in the NBA this past season. That's not a championship team. That's a team that's still trying to figure out how to win. If Durant and Westbrook can't figure out how to win those games after being together for this long, it isn't going to happen.