I would not be surprised if the Celtics deal Smart at the deadline for exactly this reason. It's not going into the tax next year specifically that hurts. It's that you're probably going to be in the tax the following year no matter what, and it's that repeater tax that really kills teams. This team is trying to win for the next several years, and you can't do that if you're in perpetually in luxury tax territory because of the repeater penalty.
If no other teams want to pull the trigger on Smart this season, the Celtics could roll the dice in the offseason, that is see what they can negotiate and also if/where that Lakers pick hits this year. If those things don't work out, then you could still sign and trade him in the offseason if you need to. As the author (J Karalis) mentioned, dealing away Marcus Morris could also help fit everything together.
From a personnel point of view, you also have to remember that Hayward is coming back, maybe even this season. He approximates some of what Smart gives you, specifically ballhandling/playmaking and a versatile and physical wing defender who can switch. The closing lineup next year will almost certainly be Irving, Brown, Hayward, Tatum, and Horford. How badly do you need Smart in that scenario, especially when Rozier will still be on the last year of his rookie deal, and you've got picks coming through the pipeline? That's the point: the Celtics will have to pick who their core players are, and since you've already got three max players plus two rookie deal guys that could easily become max players, something has got to give.
(I love Smart, btw.)