The steepness or flatness of the yield curve is an indication of how fast the economy might grow, and how much inflation it might produce, requiring higher short-term rates in the future. Well, for one, no politician can increase the fertility rate, and absent any changes to immigration policy, the growth of the labor force is expected to slow. As such, the general thinking of economists is that we'll face growth that can't change much over time, as seen in the nearby graph indicating real potential GDP, forecast far into the future.