They should keep him for one more year if they can. It is a great way to get more value from him. It could be looked upon as a delayed sign-and-trade. We get one more year of Rozier plus another team can trade for him. Sure, there are a number of teams that can pay to sign him away from the Celtics because they have room under the salary cap. However, they don't have the right to match that the Celtics hold. This requires teams to put more money in the contract to pry him away which is a fancy way to say they are betting on him outperforming his new bloated contract. That scenario won't have great results unless he becomes a MIP candidate. There tends to be a lot of buyers remorse following restricted free agency.
This means that Celtics can scare down the price, similar to Smart's recent free agency. Although more interest is expected, the aforementioned reasoning may keep the Celtics in the ball game. This could lead the Celtics to get a fair price for a starting quality point guard that can be flipped a year later to a team ready for a new point guard. Teams will pay for a quality point guard entering his prime with 3 years remaining on a deal that he will likely outperform.
This is why I stand with you when in re-signing Rozier.