You make the deal 101 times out of 100
Not necessarily.
Noel win shares last year: 3.0
Smart win shares last year: 2.9
Noel win shares/per 48: .074
Smart win shares/per 48: .083
Noel VORP: 1.3
Smart VORP: 1.1
By most metrics Smart is about as valuable as Noel. Now there's an axiom in the NBA that you want the good big man over the good little man. There's a premium on size. So even up you make that trade every time if other things are close to equal (like leadership, clubhouse personality, etc.). But when you throw in the 16 and the 23? Now the price is pretty lopsided.
I'm not saying it doesn't happen but it's certainly not a sure thing. That's a high price to pay for the premium of size. It all depends on someone's projections of player development, team needs and so forth. Personally, I'd do Smart +23 but not 16 because I see that mid 1st pick as too valuable. Those mid-1st have been Rozier, Olynyk, Sullinger, Bradley and Jefferson during Ainge's tenure. That's a pretty decent track record and I generally wouldn't include any of those guys along with Smart and 23 for Noel.
1.) Noel is, as you note, a big man, meaning his inherent value is higher.
2.) Boston needs big men a lot more than they need guards that can't shoot.
3.) Despite having missed his entire rookie season, Noel is actually the younger player, by about a month.
4.) Getting Noel would allow for the team to go for an offensive big man at #3 (or to trade for one), without worrying about how their interior defense would handle it, because...
5.) The suggested trade allows Boston to keep the #3 pick while getting a high potential big.
6.) Boston already has an abundance of picks, so using a couple of them in this way isn't any kind of insurmountable loss of options.
7.) As I noted above, Smart is a guard who can't shoot.
8.) Having Bradley means the team already has a top level defender in the backcourt.
9.) I find the idea that trading the #23 and Rozier, Olynyk, Sullinger is somehow a non-starter to be unsupportable, and the ability to get Jefferson at 15 (that's before 16, obviously, but close enough that I get your inclusion of him) was based at least partly on his being a high schooler when drafted, partly because it was a deep draft (only 4 of the top 20 picks didn't have at least 9 years in the NBA), and partly on Seattle completely blowing it by choosing Swift over Jefferson, and I don't currently see any of those 3 parts resulting in a Jefferson level impact choice in 2016. That leaves only Bradley as a real concern and, while I love his defense, I don't think that Bradley is a deal killer, at least when combining the odds against actually getting another Bradley at 16, so we'll just have to put it down to us having a difference of opinion.
I acknowledge that a significant chunk of my reasoning revolves around my questions as to whether or not Smart will ever develop an offensive game. I also acknowledge that Noel's not my ideal trade target, and any deal can come back to bite you in the ass, but I still say you make the deal 101 times out of 100.