The economist Herb Stein famously said, "If something cannot go on forever, it will stop." That's a good riposte when people wring their hands over something unsustainable. Of course, that fact doesn't tell you how unsustainable situations will stop, and some ways are less pleasant than others.
I thought of "Stein's Law" when I read former California Assembly speaker Willie Brown's response to a question about whether California's lavish public-employee pensions would bankrupt the state:
No, it's not going to bankrupt the state. My guess is that the State of California, like most places involved with pensions, is going to cease to pay them.