Your last sentence is logically equivalent to saying that no society can afford to give medical care to everyone, the only choice is who to ration it for, since

Your last sentence is logically equivalent to saying that no society can afford to give medical care to everyone, the only choice is who to ration it for, since Medicare-for-all is characteristically no different from any other comprehensive risk pool bridged over an entire population. And under conditions you're describing where the population truly is uniformly ill, every sub-population will look similar to the population at large, so no private insurance pool will do better by excluding some of the population. Sounds like a problem.