It's not a one-year deal. Most likely case is Durant re-ups with Golden State after next year when the cap raises.
Durant's situation isn't like LeBron and Cleveland. LeBron grew up in Cleveland, Durant is from DC. The Decision was literally the local kid spurning his hometown leaving for the bright lights of the city.
Durant is buddies with Golden State's Death Squad and he likes their style of play, as opposed to his fall out with Westbrook (warning: BR)
LeBron beating the Warriors almost certainly is what allowed this to happen - since it showed that villains can be redeemed in the NBA - and also that the Warriors can be as mortal as anyone in the postseason due to injuries and slumps. Golden State's vaunted well of depth ran dry at the end. The player most singularly responsible for blowing Games 5-7 is Harrison Barnes, who plays Durant's position and does it worse than him.
The biggest take home of the Durant signing isn't that he improves GSW's regular season chances. He immensely improves their postseason in part by taking out OKC as a threat. Suddenly, GS only has to fear the Spurs, and Duncan is allegedly going to retire.
OKC had its run, but really, they basically gave up the ghost by being cheap and trading James Harden. What happened in 2016 wasn't the beginning of a new era, but shining brightly before the end of one.