Dissimilarly, everyone gets old, everyone gets sick, everyone is going to need healthcare.
1. Insurance companies can invest payments from young people to cover costs when those people are older.
2. Not everyone gets old and sick. Healthy people who die in car accidents contribute much more than they take out.
3. While many people get old and sick, medical costs vary significantly and aren't very predictable for a given individual.
4. People are risk-averse. They would rather pay $1,500 every year than have a 1% chance of losing $100,000 in any given year. This is true even though 0.01 * $100,000 = $1,000 expected loss per year.
Your argument may work for old-age care, where the cost of care for most people soars off the charts for last-ditch, cutting-edge efforts to stay alive. But there is no reason the insurance model can't work for most of the population.
The x will probably change as I make more money. Right now, two million dollars is probably where I'd balk. (Pretty sad I know but I think the government could put that two million dollars to better use than to keeping me alive.)
I guess a death panel is inevitable. I am pretty sure I will change my mind and chicken out at the last minute and say "I want to live even if it costs ten million to keep me alive". Don't listen to the senile me! Let me die and use the money on what matters -- kids.
I don't have kids and don't plan on having any anytime soon.
A friend of mine uses the analogy of "food insurance" to describe the dysfunctional nature of attempts to insure against the inevitable.
No, insuring against an inevitability isn't silly because there are other factors to consider. Like when the inevitable occurs. Insurers are taking a risk that you put more in before that happens.