From Freeman Dyson:
Economic forecasting is useful for predicting the future up to about ten years ahead. Beyond ten years the quantitative changes which the forecast accesses are usually sidetracked or made irrelevant by qualitative changes in the rules of the game. Qualitative changes are produced by human cleverness … or by human stupidity … Neither cleverness nor stupidity are predictable.
Source: Infinite in All Directions, Chapter 10, Engineers’ Dreams.