Rozier has stated his desire to be a starting PG on an NBA team. He has been the only player (other than Tatum) to play in every game this season. And without Kyrie, Smart, Horford, Morris, or Brown, he has shown potential stepping up his game.
Over his last 25 games, Rozier is averaging 16.4 points (41.7 FG%, 43.6 3PT%), 5 rebounds, and 4.1 assists. 10+ points in all of 'em. In his 12 career starts, he's averaging 18.6 points, 6.3 rebounds, and 5 assists.
Next year, he'll play the remainder of his rookie contract, and then will enter Restricted Free Agency.
If the team decides to extend Smart (who Steve Bulpette says the Cs could sign for a deal in the $10-15 million range) than I can't see how the Cs would have the capspace to sign Rozier for a competetive offer, especially when it's he doesn't want to play on the bench for his whole career.
That said, he could be a valuable contributor off the bench next season. With a healthy Gordon Hayward + Theis returning, I can see this team going on a championship run if Ainge keeps the core roster intact.
A title, imo, is worth keeping Rozier for another season, even if you risk losing him afterwards for nothing.
Would you want to keep Rozier for another season, or trade him away for an asset.