1. You will get expensively sick when you get old
2. Insurance carrier will try to kick you out and not pay for hospital stays.
If getting old and having $500k operations are certain for everyone, then wouldn't a better bet be for everyone to save their money? If all of this is a certainty why have the insurance as the middle man? Save your own money, and then don't deny yourself coverage.
More realistically, what alternatives exist? There is nothing that changes the cost of the expensive operation. A single payer NHS doesn't change the amount of money operations cost.