Anything is possible. It's possible that Kevin Durant becomes the epitome of the "Ewing Effect" and OKC soars even higher without him. Not likely, but possible. 
I think Westbrook is going to impress. I think Nick Collison still has a little left in the tank and can do good things for them. I think Serge Ibaka will do Serge Ibaka things. I think Steven Adams is under-rated and will work very well with Westbrook on offense and will pair well with Ibaka on defense. Lamb could break out. Jackson is good. They are not devoid of talent--this isn't like LeBron's Cavs teams. Still, anything is possible. They could completely fall apart. They could swing a trade....maybe Phoenix decides they need size and won't have a shot at re-signing Dragic anyways so they trade him for Perkins' expiring deal, Reggie Jackson (who OKC knows they won't re-sign) and some spare parts/draft picks move around. (Just a hypothetical--odds are very small that trade happens, but it makes a little sense--Phoenix would be tough with Perk anchoring the middle and OKC would have a two-headed monster at guard)
Moving on, I am not yet sold on Dallas improving this season. (Same goes for Phoenix as currently constructed). Dallas swapped Calderon, Carter, Marion, and Dalembert for Felton, Parsons, Crowder, and Chandler. Felton and Crowder are definite downgrades (I like Crowder, but Marion is still overall a better player). Chandler is in a stage of his career where recurring injuries have set in--the bones in his legs are shot. I will be shocked if he can stay healthy. They are going to want to rest him entering the playoffs and likely won't be able to as they will be fighting for seeding. On paper, Chandler is a huge upgrade, but I have serious doubts he will play more than 50 games.
As for Mr. Parsons, he is stepping into a role that was well-handled by Vince Carter (his per36 numbers were 18 pts, 5 reb, 4 ast, 1 stl, 2 to with a healthy 39% 3fg on 6.8 attempts per game).
For comparison, Parsons' per36 numbers (which actually dip from his per game totals due to how many minutes he played) were nearly identical with 16 pts, 5 reb, 4 ast, 1 stl, and 2 to while shooting 37% 3fg on 4.5 attempts per game).
Parsons will be able to match that, but how much of an upgrade is that really going to be? Media pundits talk like Dallas just signed this offensive juggernaut that is going to fill some void. My prediction: Dallas fans will be pining for Vince Carter by Christmas. Especially when they remember that Carter played better at 1/5th the cost and Dirk's "salary discount" just got blown. Morey sends Cuban a case of red wine for Christmas with a simple note attached--"sour grapes for you" is all it says 
Again, anything can happen. I'm just not convinced that Dallas' moves are as significant as most seem to think. I think that, ultimately, they down-graded three spots and made a lateral move (at a huge cost) in the other. I would not be surprised to see them finish behind New Orleans in the standings. 
One thing is certain--this season is going to be nuts. Hyper-competitive is an understatement. The West is so good that a team like Houston, with 2 top 10 players, is an afterthought when discussing potential WC champs. The Grizzlies are almost forgotten in most discussions. I could probably construct 3 different all-star starting line-ups and not use any player more than once and each would be perfectly valid. The West is DEEEEEEEEP. I can't wait to see it all play out.