[Inziladun]

I seem to remember another recent thread that touched on this, but a quick search yielded negative results.

I think the fact that Radagast is not seen taking ship with Gandalf at the Havens is significant. It was an indication that he had totally forgotten his reasons for being in Middle-earth. R. certainly did "fail", in that he is said to have lost his focus and forgotten about being part of the resistance to Sauron, or at the least had apparently become very passive in the mission of the Istari, only doing things that were specifically asked of him by Gandalf and the White Council.
Gandalf says repeatedly that the Third Age was "his" age, and that he was the "enemy of Sauron". Since the Istari were all in the same boat, so to speak, as far as their ultimate purpose for being in Middle-earth, I would think that none of them had any business remaining behind after Sauron's fall. Staying in in the mortal lands was dangerous, for themselves and ME's less powerful denizens. There would, I think, be too much of a tendency to abuse their powers, especially after Gandalf's leaving.
I rather think Radagast, and the Blues Brothers as well were ultimately "depowered" and their bodies (which were mortal) taken away from them by their superiors. The alternative, leaving such uniquely powerful beings to their own devices after their "official" assignment was over, would seem to be a needless risk.