I'm kind of confused. What would be the incentive of keeping IT if you're not winning a championship in the next two years, and he is going to be a free agent? Are we going to let him walk and get nothing for him? Or are we going to resign him for the max?
For the rest of IT's contract, the Celts will continue winning a bunch of games since they have one of the most talented scorers in the league.
When it's time to re-sign him, they'll have a tough choice. But it seems very unlikely to me that they will have better ways to spend that money, or better options for taking the majority of the shots, during the time that IT would be under contract.
The mere fact that the team probably won't win a championship with IT as it's best player is not an adequate reason not to re-sign him, since having him under contract does not preclude one of the younger players from developing into the star we need, or prevent the team from trading assets for such a player. And in the meantime, winning a lot of games and being fun to watch is an end in itself, not a means to winning a title, and keeping IT will allow the Celts to remain in the thick of the playoff picture in the Eastern Conference.
The question is how the team should continue to maximize the team around Isaiah and Horford, not whether they should trade those two because a title doesn't appear to be forthcoming.