I think that some of you are really underrating the defensive impact of Smart. Langford isn’t in the ballpark with him defensively.
I don't think I'm underestimating the defensive impact of Smart. I think he's better than Langford likely ever will be. I think I'm downplaying the value that a defensive stalwart with an offensively questionable game like Smart has.
For example, let's say Smart is a 10 on defense and Langford is a 7 (in his better moments he was). Let's also say that Gobert is a 10 on defense and Rob Williams is a 7. Both sets of players might have a difference of 3 in our arbitrary rating. However, the difference between a good and an elite guard defender is not as beneficial to your team as the difference between a good and an elite big man defender.
The main question I have is what value that defensive difference brings to the overall team's effectiveness on the court.
This difference is also exacerbated by Smart's offense, where he is extremely prone to poor shots, bad offensive efficiency (especially if teams chase him off the 3), turnovers and ineffective hero ball.
I think the thing you aren't really looking at is quite simply offense is significantly more important to winning any given game than defense is and as good as Smart is defensively he is ever bit as bad offensively. I mean we've seen that phenomenon for the last 5 years first hand with our starting PG's. Great to elite offensive talents that are terrible defensively, yet they are net positives overall to the win column. Even Kemba is a poor defensive player (not as poor as Irving or Thomas, but still poor).
The Celtics W/L record when Smart misses games is every bit as good as when he plays for basically his entire career. He really doesn't matter all that much to actual wins and losses and I do think that is a large part because he is just so awful offensively. Defense matters of course, just not as much as offense. The saying that good offense always beats good defense is absolutely true, and that is, and always has been, Smart's downfall.
As for the trade, as I said, I have no idea who I'd take with 2, but you can sign me up for trading a terrible offensive elite level defensive role player for the 2nd pick in the draft, every second of every day. I don't care what draft it is, if Boston doesn't end up with a better player than Smart, then that is just bad drafting.