Everybody complains (with some justification) about KG's playing more and more outside, shooting a lot of jumpers, but I wonder whether that has been a conscious decision on his part in order to minimize the wear and tear on his body. It may well be that we see mostly KG's "finese" game during the regular season, and a bit more play inside during the postseason, and that's OK if it extends out his playing career by a couple of years.
With Pierce, you worry about whether he'll lose that first step at some point. He's such a good all-around player that I think he'll be effective for a long time, but the bread-and-butter of his game is getting that first step and driving the lane. He's a good shooter, but he's not SUCH a good jump shooter that you want to see him just sitting behind the three point line (the role I can see Ray Allen playing until he's 50).
Realistically, though, Ray's the guy amonng the Big Three who is most likely to be moved, if Danny does decide to make a change. I think a lot will depend on how far this group gets in the postseason; they win #18, Ray gets an extension and they go from there. They flame out in the second round, they may make a change while guys still have value. But, Pierce is the face of the franchise, he's not getting traded as long as they're competitive. That'd be like trading Bird; he wasn't going to be the one that went. KG, even as he inevitably slows down, is the guy who makes everything else click; his focus, energy, and intensity are irreplaceable.
I love Ray, but it's easier to replace a sweet-shooting 2-guard than either a Pierce or a KG.