As I understand it the main problem is that even if somebody like Gandalf or Galadriel or Aragorn did defeat Sauron using the Ring it would just mean setting up a new Dark Lord, (or Lady) in his place as none of them would have been able to resist the corruptive effects of the Ring.
Sauron is genuinely afraid of somebody using his Ring against him, suggesting that it could indeed be done succcessfuly - but talk about your Pyrrhic victories!
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