I think you're overexaggerating his value. Here are some numbers for you:
77
51
33
75
46
54
Those are the amount of games that TA has played in each season of his career. So for all the whining about Marquis being injury-prone, it's not like Tony Allen would've been any less of an injury risk.
And while he was strong, it wasn't like at 6-4 he was going to solve our problem at small forward.
And given the addition of Delonte and the possibility that Marquis can have a bounce-back year, it's quite possible that even if TA had come back, he might've lost his rotation spot.
And if Marquis can sub in for him for the next year or two, what does it really matter? He's hardly a building block for the future.
I get that the Celtics MAY regret not re-signing him, but it's far from a sure thing.