Radagast, Alatar and Pallando

[Legolas]

Though it is a pleasant thought, Gandalf would not have handed off authority to Tom, another wizard, or any other greater entity as it would've been contrary to his intent, as well as that of Manwë who sent him originally, and of Eru who reincarnated him after the fight with the balrog.

I feel like this is one of the main themes of the work. With the immortal beings fading into the West, the time had come for mortal races to grow in stature, assuming the stewardship of Middle-earth.

The Valar had tried a full-on approach with Morgoth earlier, trying "to guard and seclude the Eldar by their own might and glory fully revealed" - and well, that didn't end so well. Nor did the blunt measures taken concerning Numenor, where they dealt with Sauron on his own for the first time. This time, they were more subtle.

As above, the wizards were "forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men or Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt" (Unfinished Tales) - that is, not to fight the battle themselves, but to guide the elves, men, and hobbits to handle evil for themselves. If the battle was the wizards' to begin with, they would not have accepted the restrained forms of old men - they would've traveled directly to Mordor from Valinor at full potency and met Sauron head-on.

Accordingly, once Sauron is defeated at the end of The Lord Of The Rings, Gandalf charges the mortal races with the responsibility of Middle-earth in a conversation with Aragorn before he leaves Gondor for the last time:

And Gandalf said, 'This is your realm, and the heart of the greater realm that shall be. The Third Age of the world is ended, and the new age is begun; and it is your task to order its beginning and to preserve what may be preserved. For though much has been saved, much must now pass away; and the power of the Three Rings also is ended. And all the lands that you see, and those that lie around them, shall be dwellings of Men. For the time comes of the Dominion of Men, and the Elder Kindred shall fade or depart.'

'I know it well dear friend,' said Aragorn; 'but I would still have your counsel.'

'Not for long now,' said Gandalf. 'The Third Age was my age. I was the enemy of Sauron; and my work is finished. I shall go soon. The burden must lie now upon you and your kindred.' (VI.5 "The Steward And The King")