Going forward from when, and better how? No matter how you measure it KG was at worst one of the 2 best defensive players in the NBA last year. He scores at a good efficiency without needing the ball. And he's arguably the greatest passing power forward that ever lived. Even when he isn't the leading scorer (like he was during the championship playoffs), he still facillitates everything that this offense does and the stats back that up.
Bosh is a better volume scorer, but on these Celtics you don't want that unless you plan for Pierce, Allen and everyone else to get fewer touches. The defense takes a major step back without very much gain in team offense. Doesn't seem like a recipe for a title this year to me. Not even counting how much this kills Ubuntu for this season. Or how it affects what Allen, Pierce and Rasheed decide to do this offseason.
If you're ready to scrap the drive for 18 for the next couple years while you reload around a Bosh/Rondo/Perk nucleus then maybe the deal works. But practically speaking, I don't see how this trade helps either side because the vets on this team absolutely want to win now and would not be at all happy with this kind of trade.
Going forward from now, and better in pretty much every way except defense - and in a few years, his defense will be superior as well when KG's athleticism fades and Bosh continues to improve. Bosh may be a volume scorer, but if he wants to score 20 a game on 50% shooting then he can do that on my team. I don't care who scores the points, as long as someone scores the points and our team FG% is high. Also, how does he facilitate everything this offense does? I thought there was this guy named 'Rajon Rondo' on our team, and I thought he was a very good floor general too. I know KG does a lot for the offense that doesn't necessarily show up in the stat sheets, but Bosh has a very similar skill set and would have the same effect on opposing defenses in our system.
I will grant you that this move could lose us this year - it would take time to fit Bosh into the system. With that said, it's hard to say what effect it'll have on 'the vets'. KG is a huge part of the team, but I think they also understand that this is a business, and you took a player who has 2 years of elite play left and swapped him for a player who has 10 years of elite play left. I also think you underestimate the drive of Ray, Pierce and Sheed. KG is a big part of this team's personality, but the Celtics have a team stocked full of players who are very driven, intense, ambitious individuals in their own rights.
I like when I hear things like 'Bosh is better than KG at everything except defense, passing, and intangibles'...like that's not the majority of the game. For that matter, Bosh is also better than prime Bill Russell at everything except defense, passing, and intangibles. In other words, Bosh is a very good scorer who is average-to-below-average in those other "minor" parts of the game like defense, passing, and leadership.
And this isn't fantasy basketball. You don't get to just cut-and-paste stats from one team onto another. Bosh has averaged about 22 ppg over the last few years on about a 27% usage rate. In his last few years in Minnesota KG also averaged about 22 ppg on about 27% usage, but in Boston it's been down to around 17 ppg on 23 or 24% usage. Why is that? Because in Boston you also have Pierce, Allen, and now Rondo using a lot of possessions. So bring Bosh into Boston and either his scoring average is dropping because he isn't getting the ball as much, or the offense has to be rebuilt around him which means that the others aren't getting their touches. And frankly, they aren't going to rebuild around Bosh with the current vets we have which means that scoring-wise Bosh's contributions in green would look an awful lot like KG's. Only it wouldn't come with the all-world defense, leadership, and offense facillitating abilities that KG brings.
You ask how KG facillitates everything this offense does? He does it by being a key cog in every set and having a go-to play with all of the other starters. With Rondo they have the alley-oop and the on-ball screen/Rondo backdoor plays. With Pierce and sometimes Ray he has the pick-and-roll. With Ray he has the KG strong-side posts while Ray pops to the 3-point line with the option to shoot or pass play, as well as the pick-and-roll. And with Perk he has the high-low where he essentially forces Perk the ball anytime he is anywhere near the rim. And then, when he's not directly involved in the play, KG is out there setting picks like a crazy man both on- and off-the ball.
There's a reason that when KG is on the floor the offense has fewer turnovers, more assists, and scores at a higher efficiency than when he's off the floor. There's a reason that Big Baby replicated KG's scoring/assist averages in last year's playoffs but the overall offense wasn't nearly as efficient (even with Pierce, Allen and Rondo all playing much better than the previous year as individuals). There's a reason that KG had the highest pair-wise +/- on the team with every other member of the starting line-up. And that reason is that KG facilitates everything that this team does on both sides of the ball.
It's a team effort, it's not like it's just KG, but if you remove KG and don't replace everything that he brings (which Bosh doesn't) then be prepared for the team to take a step back. So again, if the goal is to reload and get ready to build for the future around Bosh/Rondo/Perk then yes, maybe this trade would make sense. But if the goal is to give this team the best possible chance to win #s 18 and 19 in the next 2 years then this would be a silly trade for the Cs to make.