My Grandmother, mother, and her sisters lived like that as a WWII refugee for four years. Grandmother survived The Holodomor.Some indeed will survive. But the odds are simply very bad, maybe 5-15%. For most, that existence would be a nightmare. Anyone who doesn't believe me here's a test: Cut your calorie intake to 500 calories per day. Your nutrition for the next 6 months is killing, cooking and eating rats, with your bare hands or basic hand tools. Drink only sour milk and eat only rotted spoiled veggies and fruits, in addition to cooked (or raw) rat meat, worms, small birds, etc. Drink only unfiltered water straight from your toilet bowl or mud puddles. Like your life depends on it. Stop all medical treatments, all medications, and allow yourself no hygiene and don't bathe for the next 6 months. Then report back on it, if you survive. Of course, don't do this, as you'll probably die or get extremely sick. It would be impossible to "replicate" the psychological torment of the aftermath of a nuclear attack. But just this small physical test is enough to prove a point. I doubt anyone here could do this, or is willing to "prove" they could by actually doing it. Many will make a lot of excuses why they cannot try.
They taught me a few things. I have eaten bugs, spoiled food, worms and worse. I don' take medicines so good on that one. I only eat about 700 calories a day (if I am not working), and my metabolism has adjusted to the downgrade over the years. 500 could do it for a while especially if it was graduated.
Can it be done? Absolutely. That is the purpose of prepping though, so that you can but, hopefully won't have to!