"Oh Samwise, bravest and truest of men.
Oh Samwise, Gardener of The Shire.
Samwise, you with earth beneath your fingernails
Oh Samwise, did you know what sorrow you would cause? With one bright stab of anger, one rash act born of love and fear, you sealed the fate of your master. Oh Samwise, why did you not stay your hand? Why did you not spare the creature?
Oh, Sam! How should you have known it would have spared the world?"
"If [Sauron] regains [the Ring], your valour is vain, and his victory will be swift and complete: so complete that none can foresee the end of it while this world lasts."
(Gandalf at The Last Debate.)
Gandalf was considered among the Wise, and his advice and predictions were seldom wrong. But though indeed it seemed, for a while, that Sauron's reign would endure until the last age had passed, it was not so.
The grass still grew long on the wide plains and thick in the hidden valleys of the mountain range, stubborn as weeds. Last to give way, and first to re-grow, though trampled by iron-shoes.