There are a lot of teams with enough cap space, so the odds for any given team are long.
Durant has to deal with a lot of emotions when evaluating his relationship with OKC, but he has to see the truth about their management team. Outside of Durant and Westbrook, the current roster is the weakest they've had in the Durant era. They have shed a staggering amount of talent in the last 5 years. They ruined the development of Ibaka, who they bet on as the third star. They are one of the few teams without salary flexibility, so they can't improve in the short term. and they replaced their below average coach with another below average coach.
The C's are in the opposite situation. They have great management and a great coach. They have a very young 48 win team on the rise, with defensive toughness and depth built in. They have a treasure trove of assets and plenty of cap space. If Durant was in the fold, Ainge would still have to add a couple of more talented players to our front court, and perhaps one additional reliable shooter to contend for a championship, but given the assets he has, the odds would be pretty high he could get it done. All the really hard stuff would already be in place.