[Legate of Amon Lanc]

Alfirin, I understand your line of thought, however, at least in my opinion, the in-Middle-Earth logic implies that Radagast was simply "forgotten", so to say. I would phrase it this way - he forgot about his mission and went to "play with animals", so also they forgot to inform him about his boat ticket back to Valinor, as well as about his appointment in the first place. It was not anything "evil" from Valar's part (or Gandalf's, for that matter): he did not even want the boat ticket back, obviously. It was simply like "yes, you decided not to do your duties and instead spend your days with your hobbies, we are not going to punish you for that, but we are also not going to reward you for it either; and after all, you seem happy where you are, so we simply won't interfere, you can stay in Mirkwood and be happy, we are not any enforcers." I think that's very nice and, when you look at the behavior of Eru/Valar in other situations, very typical.

Gandalf could not "give over" any authority of his. His task had been fulfilled. Radagast's task (which was the same as Gandalf's, and that of all other Wizards) had not been fulfilled. He did not do what he was supposed to do. That said, if e.g. some Dark Lord had risen in the future, nothing would prevent Radagast to stand up and fulfil his task then, if he decided that he wanted to step into the arena back again. That is, I think, also the basis for the best (and "canonically approvable") Fourth-Age-fictions. But he seemed generally not to be interested.

Let me use a parable - I think the Istari can be compared to a group of five elected politicians who are supposed to fulfil certain roles in the government. Some of them go to usurp the power for themselves, only one stays in office until the end of his term and does everything he was supposed to do. After his duty is finished, he gets some reward for that. Radagast basically quits his post and instead goes to retire in his cottage in the countryside. There are no obligations for him, he simply chose not to fulfil them. He chose not to do anything.

I cannot imagine Gandalf going to visit Radagast and "giving him over" the responsibility for Middle-Earth for several reasons. First, like I said, Gandalf had nothing left to give. Second, Radagast obviously did not want to be given any more responsibility. He did what he wanted, he did not want any obligations (maybe he did not consciously formulate it that way, but that is what he basically did). And Gandalf - especially the post-destruction-of-the-Ring Gandalf - would not "force" anything upon anybody. After the destruction of the Ring, Gandalf was just happy - at last, after thousands of years. And he also left the responsibility of Middle-Earth denizens (Hobbits with the Scouring and rebuilding the Shire is the best example) upon themselves. He would not go to Radagast and start reminding him of duties the Brown Wizard had forsaken. He would - like the Valar - just leave him do what he will.