Ok, I fully understand this is crazy and won't happen, but I'd like someone to explain to me why it shouldn't happen:

• Boston looks ready to rebuild. They're certainly not getting any further with what they've got, they don't have young prospects to trade for more vets, and there are not a lot of teams out there that could take on their vets and offer young prospects in return. I mean, they can't very well trade KG to say, the Lakers, because they hate the Lakers, but also because the Lakers can't offer them anything. And they can't trade him to say, the Kings, because KG's head would explode if he were traded to the Kings. So I think the market for a guy like KG is pretty limited.
• Dallas looks like they want to clear enough cap space to try and sign Deron Williams and Dwight Howard in the off-season. Currently, they can't do that with a guy like Shawn Marion on the books for next year, even though his salary is, on the whole, quite reasonable for a quality veteran player. And like the Celtics' prospect of trading KG, there are a very limited number of situations where it would make sense for a team to trade for a player like Marion and still offer the Mavs what they need (expiring contracts).
• We look good, but we lost to Miami last year, and they look even better this year. However, the Bulls seem to me to be one of the few teams, perhaps the only team, that could offer sensible deals to Boston and Dallas for the teams above. So... lets break it down:

Chicago Trades Carlos Boozer, Omer Asik, Jimmy Butler and picks to Boston for Kevin Garnett. Obviously this isn't a great return for KG, but if they want to move forward, where else can they trade him? The only realistic trade is to a contender, and I don't see any other contending teams out there that could reasonably make a trade giving them anything back. On the other hand, this trade gives them a center prospect, picks, and a pretty good PF that makes 60% of KG's salary and who will play fairly well for the rest of his contract. A team with Rondo, Pierce, and Boozer is probably still going to make the playoffs (I'd assume they trade Allen and bring in some more prospects as well), but it's way younger, and has potentially found it's center of the future in Asik.
For the Bulls.... do I really need to explain why KG is an immense upgrade from Boozer?

Chicago Trades Kyle Korver and Ronny Brewer to Dallas for Shawn Marion and Roddy Beaboi (and Ian Mahinmi because here's hoping to them being really desperate to be under the cap next year). We turn our average bench players on expiring contracts into two really kick ass bench players on slightly longer contracts. Dallas still squeaks into the playoffs, but they've now got another $10M to work with next summer. Who else is gonna do that for them? Nobody, that's who.
The whole trade appears to work
Why should the Bulls do this?

• Because they aren't going to pay the luxury tax to re-sign Omer Asik. But they might to add two NBA champions (and a sure fire hall of famer) who can still play.
• Because a team of Noah/KG/Deng/Rip/Rose with Taj, Marion, Beaubois, and Watson coming off the bench would be impossibly good, probably even the odds on-favorite to win the title this year. Yes, KG is old, but we can reasonably rest the hell out him with Taj and Marion, and hope to get another two years of high quality play from that group. To spell it out, that's three years of likely NBA titles.