If something cannot go on forever, it will stop.

Herb Stein (1916-1999) American economist

Stein’s Law (1980s)
    (Source)

This was a frequent statement by Stein. He explained it so:

This proposition, arising first in a discussion of the balance-of-payments deficit, is a response to those who think that if something cannot go on forever, steps must be taken to stop it -- even to stop it at once.

Variants:

• "If something can’t go on forever, it won’t."
• Anything that can’t go on, won’t.
• Trends that can't continue won't.