He was at a high level at the end of '15. He's done so much here, but that series against the Clippers is one of my sentimental favorites. His performance against those two monsters was absolutely heroic - the aging lion. He deserved to win that series, and would have if he had gotten any kind of support from most of his teammates.
If he had been able to play at that exact level this season, there is no way the Thunder win that series. So, no, he shouldn't have quit after the '14 season, because he was still a beast in the '15 season.