Beyond all the financial restrictions which make the likelihood of this not happening stronger, there are plenty of other things to factor into it. Wade plays for a warm weather team down in one of the biggest party cities in the States. Unfortunately these things do really weigh in a players mind when they are deciding what they are going to do in the future. Just the Boston weather alone would make it unlikely that he would be jumping at a shot to join a Boston squad.
He's already won a championship, and he knows how great that feels. I'm sure he wants another one, but he's still young and has time to start over on any team he wants essentially. He's not some veteran who's championship starved nearing the end of his career. He will have lots of suitors and contending teams looking to acquire him.
I'm not saying I would ever reject the idea of Wade being on the C's. Of course I would want him, but would he really be the ideal guy to replace Ray Allen? I think Wade's game isn't exactly suited for what the C's do (share the ball, work in and out of the post, hit open shooters if defenses collapse, and finally go to the one on one Pierce style option if those other things aren't there.) I think we need someone who is more like Ray Allen than we do someone who is like Dwayne Wade, who is definitely not known for his jump shooting or 3pt shooting efficiency.
In the end I think the best team for him is the one he's on now. They will be able to offer him the most money, a warm crazy city to play in, the sole responsibility as star of the team, and continuity in his career. That's just my opinion though.