As this season has proven, injuries happen and its best to have good players on that bench to fill in when others go down. Next year has to be an all in for a run at the title, so you keep Rozier. He's turned himself into an impactful player and he's cheap so you keep him for next year, irregardless of what Danny does Smart.
If the Celtics win the title, then its time for management to decide how much the want to go into spending money to keep that championship core together. That's when its decision time for Rozier, not this offseason.
The tradeoff is that you have Rozier in case a guard is down for the playoffs, but if no one is seriously hurt, either Smart or Rozier will not play much at all. If you end up wanting to move Rozier after 2018-19, he will be worth quite a bit less. He will likely have played much less, and we will have used up one more year of his rookie deal.
I just don't think the insurance is worth squandering that much value.
So you assume, Smart signs, Kyrie, Brown and Smart never get hurt, each play all their minutes at guard with Brown and Smart never playing the 3 in a 3 guard lineup and Rozier only playing mop up minutes. You also assume, that just because Rozier barely plays that suddenly GMs across the league will just forget about Rozier's 2017-18 season and suddenly won't want to sign him or sign and trade for him at any point.
Yeah, I am going with Rozier is around next season, he plays a bunch of minutes every game, he gets better and better and he helps the team to their 18th championship. Then he gets resigned and the team decides to play lots of luxury tax to keep the Irving, Smart, Rozier, Brown, Tatum, Hayward, Horford core together while drafting bigs and signing bigs to help fill out the roster