The basic idea is that reality or truth is better than illusion. Which seems to close to what "Menagerie” is about at the end of the day - no human would choose the illusion, or should, no matter how pleasant. But that’s where it gets mucky, because in “Menagerie,” they put it explicitly in terms of pleasure. Delusion is tied to captivity pretty closely, and humans are hard-wired to dislike that so intensely, even a very comfortable captivity would be so unpleasant for them, they’d rather die. We choose reality not because it’s better but because we’re somehow hardwired to find it more pleasant.
Which isn’t Plato’s point at all, I don’t think. Maybe the people in the cave are happier than the people out of it, even after they’ve adjusted; they’re still worse off. But how the heck do you make that point - what possible way do we have to compare experiences beyond what’s pleasant to us, what we’d choose? And how do you get a fair comparison? The only people capable of comparing are the ones who know they’ve been deceived, which has to sour how pleasant they find life in the menagerie. The fact we can’t imagine a story that breaks this goodness == pleasantness barrier is a pretty damning criticism to my mind.
Also loved the female first officer in the flashbacks. Didn’t I hear somewhere that Roddenberry wanted to go that route for the main series but the studios nixed hr being a woman and we got Spock instead? There's something ironic there: take away that progressive route, and we get (1) a two-parter starring her anyway, and (2) a biracial character who (3) gave us so much queer subtext the fandom and society at large is still wrestling with it in some fashion or another. The moral arc of the universe is slow, etc.
Though I must say, the way they dealt with ugliness and disability? Kind of turned my stomach. Suppose you can't have it all at once..