I think a back-up bearer would have been fruitless and ended in failure. As Elrond in his infinite wisdom says:
Quote:
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"I think that this task is appointed for you, Frodo; and that if you do not find a way, no one will."~The Council of Elrond
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Even if someone else would have had the strength to lug the Ring all the way into the Sammath Naur (which Tolkien writes in a letter saying Frodo was possibly the only one
ever who could have done such a thing...but that is for a different discussion), Frodo's own redemption came when he spared Gollum's life, when everyone else would have told Frodo to kill him. Certainly not Sam, nor do I think any of the others, would have found pity in their heart to spare Gollum.
If a back-up bearer had to be chosen, the best I guess would have been Sam...afterall he did prove to be a trusty temporary "bearer." But in the end, Frodo's death would mean the death of the quest.