I could look it up in Letters, if my copy was to hand, but Tolkien wrote (IIRC to Eileen Elgar) that NO one could destroy the ring at the Sammath Naur. It was beyond the strength of will of any, even a Gandalf. Frodo (and by extension Isildur) cannot be faulted for not doing the impossible.
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The entire plot of The Lord of the Rings could be said to turn on what Sauron didn’t know, and when he didn’t know it.
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