The thing about basketball is that you only play 5 players on the court at once and in the playoffs, you can get buy with an 8-9 man rotation. Thus, unlike others sports like baseball and football that require a lot more depth, basketball is very star driven. I mean think about it, Brady plays less than half a game. Ortiz only bats 5 times a game or so. There's nothing Brady can do to help the Pats if the other team has the ball. And there's nothing that Ortiz can do to help the Sox if he's not up. On the other hand, as the playoffs showed, stars can play virtually the entire game (Allen did at least once) and contribute offensively and defensively on practically every possession. Thus, when you have an All Star, you don't trade him for two decent players.
There's been countless examples of this going badly. The Barkley deal, the Chamberlain deal, and more specifically the two trades that landed Allen and Garnett in the first place in green. The Celtics certainly got the better end of that deal in the short term, and with the age of Pierce and Garnett, the short term is all that matters.
More importantly, where do Diaw and Barbosa even fit in? Rondo is better than Barbosa, so Barbosa's not going to get much time at the 1. Diaw's been a beneficiary of Steve Nash and is only marginally better than Posey.
It's a terrible deal any way you twist it. Stop thinking about the future, because it's going to ruin the present.