(Continued, due to post-length limits)
There are a lot of passages describing the danger of the Ring (primarily the danger of Sauron recovering it; secondarily the danger of someone else wielding it and becoming a Dark Lord), and discussing the possible courses of action to avert the dangers. All the discussion and the courses of action are aimed at denying Sauron the Ring. There is no statement from anyone that destroying the Ring will destroy Sauron or result in victory. It is very hard to come to a reading that participants in the Council think that destroying the Ring will destroy Sauron. It is very hard to see how a first-time reader could assume that destroying the Ring will destroy Sauron.
I guess it depends on how you're reading the text. If you're only doing a close reading, and only considering what is explicitly mentioned in the text, then you are correct; but I feel you're then likely missing the forest for the trees. If we take a step back and think about what we know and have been told, I think we can see it somewhat differently.
We
know that the Free Peoples must not wield the Ring (from Boromir's pitch and rebuke).
We
know that if the Ring is
merely hidden and unused, Sauron wins (from the discussion about Tom Bombadil).
Therefore, we can conclude that the Free Peoples cannot win through fighting strength alone, and if the Ring's destruction
merely denies it to Sauron and accomplishes nothing else, Sauron wins.
We
know that there are incredibly wise people in the Council who should know this much or be able to easily figure it out (Erestor excepted).
We
know that not everything talked about in the Council was explicitly recorded.
We
surmise that the text (from within the story itself) was written by multiple individuals, and may be limited by their own knowledge and memories.
From these, I see three possibilities:
1. The Council knows only the information they have explicitly mentioned, and have still decided to take the enormous risk of sending the Ring to Sauron's territory without any idea of actually accomplishing anything.
2. The Council (or at least some of the Council) know that destroying the Ring will accomplish something beyond merely denying it to Sauron, but this was either discussed outside of the Council ahead of time, or was known/assumed by enough present without discussion.
3. The Council did, in fact, discuss this, but the author either failed to record it or misunderstood the arguments.
Of these three, I reject #1 outright. If they do not know or suspect that destroying the Ring deals an active blow to Sauron, then destroying the Ring accomplishes nothing except increasing the odds that Sauron will reclaim it. If Sauron can beat them with the Ring hidden, he should also be able to beat them with the Ring destroyed, if the two options are equivalent. This decision would make the Wise out to be morons, which is inconsistent with their portrayal in the text. If this were a children's book with absurd humor, I could buy that; but in an epic where some characters are consistently portrayed as having good sense (even if not infallible), it's conspicuously out-of-place. Given multiple possible readings, if any of them are completely inconsistent with the text as a whole, I feel I must reject it.
#3 is possible, though I feel that relying too much on fallible narrators to explain away potential plot holes runs the danger of becoming a cop-out. For that reason, even though it's possible, I would not personally use it unless there were no other options.
So, #2 is the reading I feel fits best. It may not have lots of explicit support from the text, but it is, I feel, consistent with it (at any rate, moreso than the others).