Buy, Smart might not be the player Dunn is but he's close enough that the value of taking the less skilled but also less redundant Brown puts it over the top. Dunn is too similar to every other guard in the rotation and we would still have nothing developing behind Crowder.
Then there were the early reports that Dunn's camp didn't want him to be drafted by Boston. They later put out a statement claiming that those reports were false but IMO where there was smoke there was fire, doubt he would have had any interest coming to our crowded rotation. Jaylen wants to become a top player and do so spending the foreseeable future with Boston.
Wouldn't be hard to picture a world where Dunn would start demanding 1 of He, Smart, Bradley or Rosier were traded. Our logjam bursts and we have to move one of them for peanuts since every gm we deal with knows we aren't bargaining from a position of strength.
You'd do that for a top young recruit like John Wall when he came out but if Dunn is only a mild upgrade over Smart or Bradley you just burnt off 2 of some of your better assets (traded player and 2016 Brooklyn pick) for only a minor upgrade bordering on lateral.