Seeing how Bird, Parish, and McHale declined and Ainge's comments on how the Celtics should have traded the latter two seems to have made some people overly paranoid about having aging stars on the team who aren't in the category of life-long Celtics who you can't bear to see in another uniform (Bird, Pierce). Ray Allen has the skill set where he might be a quality NBA starter for more than just 2-3 years more.
Ideally, the Celtics would like to wring value out of those 2-3 years when he is very likely going to be near All-Star level and then trade his name value and last few years for draft picks or younger players.
Instead, what the Celtics need to think about is configuring their bench to fit the flaws of Allen's skill sets. As the worst defender in the starting lineup, Allen is going to be one focus of offensive schemes. It wouldn't be a bad idea for the Celtics to have a one-way defensive stopper who might play in the place of RA at the end of close games with a small lead (but you would keep Ray out there when trailing because you want his offense), someone who you might swap with Ray Allen on offense and defense when there are a lot of time outs and fouls and other stoppages in play in the last minutes of a tight game.
The Celtics wanted Tony Allen to be that guy, but he wasn't. It doesn't have to be some Posey-like super-sub who can soak up a lot of minutes at multiple positions. It might be a better allocation of resources to find the SG equivalent of Sheldon Williams for the minimum.