Ainge has to ask himself how necessary Posey is to repeating next year. If he deems that the Celtics need him to have a reasonable shot at #18, he must sign him regardless of the length of the contract. The window is closing on this team to win more titles.
Forget the future. Remember, only 1 team in 30 wins it every year. In every era there are tons of teams that build winners but can never get over the proverbial hump (think the Hawks and Bucks in the '80s, the Suns, Knicks, Heat, Blazers, and Jazz in the '90s, and in more recent years the Kings and likely the Mavs and Suns). To risk not winning a title over the next couple years so we are better able to rebuild in 4-5 years is not playing the odds very smartly. Even if Ainge does rebuild and does it well, it's far more likely the team becomes the late '90s Jazz than the late '90s Bulls (actually, it's more likely the C's become a mediocre team like the late '90s Blazers). That's no knock on Danny, just reality.
We really need to keep that Carpe Diem mentality and get everything we can out of this team.