I think this is a mistake by Ainge. Garnett's here for four more years as it is. I can't imagine that Pierce doesn't get extended to at least the end of Garnett's deal. Rondo is going to get a pretty sweet long term deal at some point. So I don't know where Danny realistically thinks that he's going to need cap room in the next 4 years. Moreover, banking on cap room in 3 years doing something for the C's is a long shot at best. The premier free agents will likely re-sign with their current teams; furthermore, in 3 years, the appeal of playing with Pierce and Garnett isn't going to be what is is now as both will be in their mid 30s. On top of all of that, if Danny wants to make a big FA run, there's no reason why that can't happen in 2011, rather than 2010. So in short, there was no reason not to give Posey 4 years other than the fact that he might have been declining by the end.
That said, hopefully Ainge can do something to help fill Posey's shoes. Darius Miles intrigues me. He has the ability to both play and guard most positions on the floor. I worry about his shooting range, though. One of the big pluses with Posey is that he could space the floor very well.