Lebron will likely finish top 5 all time if he wins a couple more rings, Durant will finish top 10-15, maybe break the top 10 with a couple rings himself. OKC fans should be very happy and grateful that they got Durant. Portland probably hates their life right now .
Durant though deserves MAJOR respect. I remember 3 years ago rookie Griffin was compared to Durant and I argued that Griffin had more long term potential due to being a big man and performance rookie years. That was back when most of us non Thunder fans considered Durant a 6'11 Ray Allen who was mostly a pure scorer.
Now we are 2 and a half years out and Durant has become a very respectable defender, a good playmaker AND a very very good rebounder. The all around improvements to his game are crazy because I never saw them coming. It just goes to show how freaking hard he works and how amazing of a player he is. To think the guy is just 24 years old too? It's unfair for the rest of the NBA because I have no clue how much better he still can get.
But once LeBron declines, he will have 3-4 years where he'll be in good position to be the undisputed best player in the league (which, ultimately, matters more than imaginary rankings). Durant these past few years has become the Nadal to LeBron's Federer.
I think he does personally (as do a few other guys), but that Finals loss will likely be a irreconcilable blemish on his resume in the eyes of a lot of fans. Not that I agree with it personally, but the 6/6 record in Finals is a big part of why people have Jordan as the GOAT on their lists, and it doesn't seem they're going to move away from it.
He's 24 years old, who knows how the rest of his career plays out. The all time rankings really isn't a place for players still in the their primes in my opinion. We'll be able to properly rank players like Lebron and Durant after seeing their full body of work.
Supposing everything in his career goes great for him - playing (and staying) for a great team for his whole career and winning multiple championships, winning more scoring titles (which is the only type of statistical title that counts for some fans, after all...), getting his share of MVP's, staying healthy till old, LeBron's career for some reason not keeping track, lack of other all-time great opponents in his own era, I don't see why, career-wise, he wouldn't be mentioned among the GOAT. This part is partially a matter of luck.
Individually, he doesn't have the multiple sector impact than players like Wilt, Kareem, Jordan, Magic or Bird had, so that would be a hit, but, if everything goes right, it's a given that he'll be discussed. The #1-3 best of every generation since the early 60's are discussed among the GOAT and I don't see why the 2010's will be different.