Durant's legacy will be defined by the direction the NBA is going to take in the next few years. He might be the worst, but not the first. LeBron didn't join the team he lost against the previous year, but Miami instantly became the best team on the planet, by far in the East. And he probably wouldn't have gone back to Cleveland if they didn't offer him a good package there either.
Ultimately the league has to decide whether they want this trend of super teams to continue or not. If not, the rule that prevents this may very well go down as "Durant rule", and he's going to become the laughing stock. The one player that won championships the easy way as long as it was possible.
On the other hand, in case the current system prevails and generates
revenue / ratings, he could become the leader of a new era.