Forecasting trend growth is difficult because, although it's based on long-term averages, the growth rate itself is not fixed. Some drivers, like aging populations, change slowly and predictably. Others, like unexpected global crises or technological disruptions—known as exogenous shocks—are sudden, unpredictable, and only clear in hindsight. These shocks can significantly shift economic growth trajectories and aren't fully priced into assets beforehand. Therefore, understanding and anticipating changes in trend growth is crucial for making informed investment decisions over the long term.