As DA said, there is once in a while someone available that can change the face of a franchise. And he wants to be sure he has assets when this time comes. Meanwhile he's trying to add value in term of bargaining chips (draft picks), productive players and cap space.
To me, this year has been better than expected for him, as he add another value to this team, Brad Stevens. Now the entire nba talks about him and start to mention him as COY.
Top players who see and understand Celtics winning culture and history and in the mean time are not comfortable with their situation might suddenly give a thought to become a Celtics.
Who knows, maybe KD will be available next year. Even though it's highly unexpected. But KG didn't expect to leave Minnesota either...