Is Plato saying that it's hopeless to free them or help them to see the truth.
No. He didn't say that. He said that well trained guardians who had learned to see things in terms of the metaphorical "light" shed by THE GOOD (similar to the light of the SUN in the physical world) could be justly compelled to go back down into the metaphorical cave, let their eyes get re-accustomed to the darkness, at first, and then explain the shadows to the cave dwellers, better than cave dwellers can. Quote:
Wherefore each of you, when his turn comes, must go down to the general underground abode, and get the habit of seeing in the dark. When you have acquired the habit, you will see ten thousand times better than the inhabitants of the den, and you will know what the several images are, and what they represent, because you have seen the beautiful and just and good in their truth. And thus our State which is also yours will be a reality, and not a dream only, and will be administered in a spirit unlike that of other States, in which men fight with one another about shadows only and are distracted in the struggle for power, which in their eyes is a great good. Whereas the truth is that the State in which the rulers are most reluctant to govern is always the best and most quietly governed, [Republic Book VII]
Plato/Socrates's point is that in his ideal Republic, people with philosophic "natures" won't be spoiled but will be educated in a spirit of service to their fellow cave dwellers. But they will also enjoy one another's enlightened company much better than the company and pleasures of "den dwellers". So when they are compelled to go back down among cave dwellers, they won't be seeking the shadowy "honours, rewards and pleasures" of cave dwellers, but, rather, will be showing/explaining the "shadows" of cave-dwellers to them as a service. Of course, if some "shadow watcher" wants to climb up to the light of philosophy, they would certainly help him/her in achieving that desire.
Basically, Plato was preparing his "fellow philosophers" for rejection and misunderstanding among "shadow-watching-people" while reminding them that it was their duty to help such people understand their "shadows" better --- or, if they desire, help them up toward the "light" of being virtuous and good.
As to "our knowledge" [i.e. So what is the point of the story if we can't use OUR KNOWLEDGE to free other minds?], why don't you know the "point" of Plato's allegory. It is written above in Book VII of Plato's Republic. Could it be that you are still a bit of a "shadow person" or "cave dweller" yourself and somewhat unable to get the point of an allegory or, as you say, "story"?
I don't mean to be unkind with the above question. Luckily we are not talking to each other in the same "cave". Hence I should be "safe". Just kidding.
"Yes, my friend, I said; and there lies THE POINT. You must contrive for your future rulers another and a better life than that of a ruler, and then you may have a well-ordered State; for only in the State which offers this, will they rule who are truly rich, not in silver and gold, but in virtue and wisdom, which are the true blessings of life. Whereas if they go to the administration of public affairs, poor and hungering after the' own private advantage, thinking that hence they are to snatch the chief good, order there can never be; for they will be fighting about office, and the civil and domestic broils which thus arise will be the ruin of the rulers themselves and of the whole State."
So don't embroil yourself with "leading people out of darkness" with "OUR KNOWLEDGE" until you are able to "explain the shadows" to those of us who, most likely, also live in the metaphorical "cave".