This must be the first episode in the entire "Twilight Zone" series where I can't really figure out whether I liked it or not. In fact, it left me stone-cold, and that obviously also isn't the kind of emotion or sentiment you seek for. What is good (albeit familiar) about "The Old Man in the Cave" is the post-apocalyptic setting, and mankind's realization that - ten years "after the bomb" - they only brought misery, suffering, and despair upon themselves. What I didn't like at all is the exaggerated preachiness of the script. Now, many TZ are preachy, but this is - to my knowledge - the only episode in which Rod Serling blatantly suggest a tragic outcome for people who don't listen to the preaching.
Sounds confusing, I know, but I promise it makes more sense when you watch the episode. The preachy aspect can be summarized as: you need to believe in something, and you simply must hang on to your belief no matter what. The inhabitants of a small town managed to survive reasonably well during ten horrible years in the aftermath of a nuclear disaster, primarily because they blindly followed the advice of an unseen deity. The so-called "Old Man in the Cave" spread messages via the community leader, like "the radioactive wind is blowing from the North" or "the canned food is also contaminated", and they adapt. With the arrival of a skeptical and brute military squad, the townsfolk begin to question the almightiness of the wise Cavemen, and suddenly their chances for further survival deteriorate.
So, what are you implying, Mr. Serling? That you can never question or challenge the things you always believed in before? That you must always follow the masses like sheep? I don't feel cozy with morals like these, despite the episode benefiting from an uncanny post-apocalyptic atmosphere, and the presence of James Coburn.