Let's check back in on this after 20 games. But, I agree that Rozier is balling out and he could give the Celts some useful leverage in negotiating with Smart.
Since both Rozier and Smart can play off-ball and Kyrie has a colorful injury history, I think it would be nice to have both of them on the roster, but not if Smart wants starter money.
On the other hand, regardless of how well Rozier is playing, it would be tough to see yet another 1st round pick walk out the door after 4 years. That's not a great way to protect your assets.
Since the Celts will be capped out anyway, if they think Smart's value is unlikely to go way down immediately after signing a long term deal, they might as well sign him even if the plan is to look at shopping him at the following trade deadline or offseason.