We went 7 games. All you have to do is win one of the 4 we lost. Any of those players does that. Jeff had it on the main board that Joe Smith said he wanted to come here. Birdman was out of shape, but was still in better shape than O'Bryant, who was also out of shape. Barnes also would have given Pierce fresher legs. Just needed one more quality player (Posey or any of the guys I mentioned) and we would have gotten by Orlando. Powe even. Now Cleveland, we probably needed KG. But we can agree to disagree on whether Posey would have gotten us by Orlando. I say yes and you say no and that's fine. Next year we can have this same discussion about Baby 
I think whether or not Posey would have gotten us by Orlando is of less relevance than whether or not he would have won us the title.
Besides, all these things were gambles, and with 20/20 hindsight it is easy to second guess.
No one predicted that the Birdman would have such a season, and there are no guarantees that he would have played that way outside of Denver's system.
As far as Posey goes, we would have overpaid for a 32 year old that really would not have won the title for us this year.
And with relation to Smith, first of all there is no evidence that he said he wanted to come to boston, second, even if he did there is no evidence that is sincere, third, there was no guarantee that he would be released, and finally, regardless of what he wanted to do, even before we signed Marbury Cleveland had more of its MLE left to sign him.
Whatever we think of Ainge's results this season, at least he tried. He tried to get Maggete, he tried to get Posey, he tried to get a big and he tried to get a back up PG.
We could just as easily be here complaining about how in trying to go for Smith we ended up without a back up center, and had we signed Posey we could just as easily be here again, bumped by the cavs as opposed to orlando, complaining about the lack of flexibility because we had Posey at a significant amount for 3 more years.