I'd pass on Duncan. He's the type of player you hope to get lucky with, not the type you give up assets for when he's in the middle of a down year just to have the privilege of paying him $18 mil for the next 4+ years. Strus would be the guy to trade Smart for, but the contracts obviously don't line up.
I grew up 15-20 minutes from Duncan Robinson so I'm a bit of a fan. If they did trade for him, I'd learn to like it. I worry how he ages though. While shooting is the last skill to go, the rest of his game can't really afford any type of declines.
Getting Duncan Robinson for Smart also REQUIRES making another move for a PG (at the very least, someone who can bring up the ball), regardless of what the team does with DS. I would heavily prefer Huerter if that were a real option. While not a PG either, Huerter could probably fake it for a few minutes a game and I like his overall game more. He's also 4 years younger.
Robinson would be a gamble. The C's would be getting rid of one of their more valuable trade chips in the hope Robinson will rebound. If he gets back to 40%+ from 3 on 8+ attempts a game, it'll do wonders for the Celtics offense, especially Jaylen Brown. I could also see TL benefiting a lot with no MS and a return to form from Tatum and DRob. TL/Tatum/Jaylen/Duncan/xxx is a very intriguing lineup If Tatum/Brown/Duncan are all hitting 38%+ from 3 on around 8 attempts a game. 2 of TL's biggest strengths are offensive rebounding and his passing/vision.
Ideally, xxx would be a PG who can shoot the open 3, doesn't need a lot of FGA, and has better playmaking skills than DS and Smart but with TL/Tatum/Jaylen/Duncan, the team could get by with a non shooting PG. DS would be an ok fit with that core, though I think he'd still be better coming off the bench where he'd be the 2nd scoring option.
I dunno, maybe I would trade Smart of Duncan if I knew Duncan (and Tatum) would revert back to form. I see Jaylen Brown and TL thriving in that situation, but Tatum would benefit too. Whoever the 5th player is could drastically change the look of the team too. Go with PP and you have 4 deadly shooters and TL on the court.
Seriously though... adding Duncan to this team would completely change the identity. On paper, it would go from a meh shooting team to an awesome one. Tatum, Brown, Duncan, PP, JRich, Grant Williams. One would almost have to go out of their way to not have enough shooting on the court. Outside of the 2 big lineup, anyway. One would think if the C's acquired Robinson for Marcus Smart, it would be the death of the 2 big lineup. I can't see Ime going with TL/Horford/Jay/Jay/Duncan and I can't see trading Smart for Duncan if Duncan wasn't going to start.