Stein's Law
If something cannot go on forever, it will stop*.
If a trend cannot continue, it will stop.
Herbert Stein (1916-1999) was a senior fellow at the American Enterprise Institute chairman of the Council of Economic Advisers under President Nixon and President Ford. He is the father of actor Ben Stein.
*One of his examples was the rising amount of U.S. balance of payments deficits. He argued that since this cannot go on forever, there is no need for a program or policy to stop it, since it will stop eventually on its own accord.