I think you have to be careful about using cap space as a reason not to make a trade (in general, not just this trade). Cap space only means you have a chance to sign someone. There needs to be some one to sign and that person needs to want to come to Boston. I don't foresee a big signing in the next 1-2 off seasons anyway. We are going to run with Thomas, Horford, and whoever we can put around them. That duo is not going to win a title; there are not going to be any Durants to sign. We might as well trade for someone who makes us better and is on a fair contract that runs for that shorter window.
Right. It is sort of like a "pretty good bird in hand" vs "an outstanding bird in the bush"
I think the argument is that to be a championship level team, we need the outstanding bird, so let's keep hunting for it.
Certainly Vucevic, or someone of similar quality, could be acquired next summer if nothing else materialized.
That outstanding bird may be Brown. It may be who we pick with the BKN 2018. Waiting to sign the outstanding bird is one option but not one that has a track record of great success rate.
Signing the outstanding bird has at least as good a success rate as waiting for one to develop. The team is said bird away from championship contention as presently constructed. Brown isn't going to be that player next year (and I really like Brown a ton, such that his jersey was the first Celtics apparel I've purchased since the rebuild started.) The 2017 pick won't be that player next year. And the 2018 pick by definition won't be.
Those are all great pieces to have if we're unable to get someone now (and I'm not in favor of trading any of them away for all but about 15 or so players). If we miss out on a trade this year, a free agent next summer, and a trade at the next deadline, then they point to a future that could still be bright as players like Bradley, Smart, Thomas, and Horford leave (I'm not saying all of them go, but I expect at least one of them will before Brown and the other Brooklyn picks completely blossom.)
But the plan should be (and I believe is): Get that outstanding player. Then fill in any last remaining holes (that may be a big, but it may not depending who is acquired). If that player can't be found, then the goal is to keep this core together as much as possible, and mix in the youth as they become ready. And, as needed, swap a veteran out for a veteran at a different position if the youth pushes through at one position but not another.
I would be very surprised if Vucevic, or someone of similar quality and contract status, were traded for this season without us acquiring someone like Jimmy Butler first. I would be less surprised if the trade were in the summer, and even less at next year's deadline.