You think that the Celtics are going to re-sign him after next year? It seems very plausible to me that they want to try to get rid of him for some type of return now if they aren't interested in re-signing him.
It is pretty sure they do not want to resing him, but they are not gonna trade him now, either. Sure, Allen is getting old and he is inconsistent, but they will give the Big 3 one more chance. KG's knee gave up this season, so they hope he will be able to return as strong as he used to be next year. They still think they can repeat that championship with all those guys healthly.
After the playoffs this year when a clearly past-his-prime Ray Allen disappeared for long stretches of games, the Celts would be crazy not to shop him. He will be 34 years-old this summer. He barely shot 40% from the field in the playoffs. It's no sure thing they trade him, but Ray Allen isn't indispensible. If he had played up to snuff, the Celts would have won the Orlando series.