[Raynor]

Even though they were great fighters, I don't think the sons of Elrond with a few Dunedain could defeat a force that outnumbered them by the thousands

That is a wrong idea to start with; it is stated throughout the Last debate what the purpose of this _would be suicide_ attack is: a bait, to divert Sauron's attention:

Victory cannot be achieved by arms, whether you sit here to endure siege after siege, or march out to be overwhelmed beyond the River
...
His Eye is now straining towards us, blind almost to all else that is moving. So we must keep it. Therein lies all our hope. This, then, is my counsel. We have not the Ring. In wisdom or great folly it has been sent away to be destroyed, lest it destroy us. Without it we cannot by force defeat his force. But we must at all costs keep his Eye from his true peril. We cannot achieve victory by arms, but by arms we can give the Ring-bearer his only chance, frail though it be.
...
We must push Sauron to his last throw. We must call out his hidden strength, so that he shall empty his land. We must march out to meet him at once. We must make ourselves the bait, though his jaws should close on us. He will take that bait, in hope and in greed, for he will think that in such rashness he sees the pride of the new Ringlord
...
We must walk open-eyed into that trap, with courage, but small hope for ourselves. For, my lords, it may well prove that we ourselves shall perish utterly in a black battle far from the living lands; so that even if Barad-dûr be thrown down, we shall not live to see a new age
...
Indeed the force that we lead east need not be great enough for any assault in earnest upon Mordor, so long as it be great enough to challenge battle.

They went to their almost certain death, because this is what they perceived was their duty - or else, no new age shall be.