I’ve always appreciated the insight of “Original Sin” in the sense that we are all flawed.
When I was an anarchist I was utopian. Then I moved to libertarianism (which I regard as utopian). I saw a lot of it in university but even since then I still see lots of people who may not outwardly profess utopian ideology but who fall for utopian-type campaign promises (some problem is going to be eliminated permanently) or who, deep down, believe in the real world possibility of certain types of perfection (be it equality, freedom, justice — the last one is everywhere).
I don’t see Plato as saying that we just need to have an image in our mind before we do something but that these ideal images exist and need to be accessed for us to make something or achieve something. I know a lot of people disagree with this interpretation and try to claim there was nuance there, but I think a lot of the nuance attributed to him is hindsight from our modern perspective. That’s just my opinion though. I’m not a fan, as you probably could tell.