If something cannot go on forever, it will stop.
Stein's law is a warning about extrapolation, formulated about US trade deficits. Unsustainable trends end whether or not anyone can say when, which makes "this cannot continue" a weak prediction.
In practice
- A firm's costs have risen faster than its revenue for six years, and the trend ends whether or not anyone in the building can say in which quarter.
- Its weakness is that it forecasts nothing: "this cannot continue" is compatible with continuing for another thirty years.