On a longer time scale, the plot thickens:
It almost looks random. A large drop in real rates in the mid-70s, a massive spike in the early 80s, followed by a slow multi-decade decline.
The chaos doesn't seem to be due to interest rates. They steadily climbed and steadily fell:
All that's left is inflation:
First, real rates should be expected to pay a moderate rate, so nominal rates will usually be higher than inflation.
However, interest rates are very stable over long time periods while inflation is not. Economists call this type of phenomenon "sticky pricing", where the price, interest rates in this case, do not change much despite the realities surrounding them.
But the story is a little more complicated.
In the early 1970s, Nixon had an election to win and tried to lessen the impacts of recession by increasing gov't spending, not raising taxes, and financing through the central bank, causing inflation. The strategy failed, but he was reelected anyways.
This set the precedent for the hyperinflation of the 1970s that ended abruptly by Reagan at the beginning of his first term in the early 1980s. Again, interest rates remained sticky, so real rates spiked.
Now, the world is not growing, almost stagnating. Demand for equity is somewhat above average, but because corporate income is decelerating, and the developed world's population is aging, demand for investment income is skyrocketing.
As demand rises, so does the price, which for an investor is a form of inverse of the interest rate.
Future demand is probably best answered by forecasters, and the monetarist over and undertones still dominating the Federal Reserve show that they have finally learned after 100 years that inflation is best kept "low and stable":
But what happens if growth in the US suddenly spikes, inflation rises, and the Federal Reserve must sell all of the long term assets it has bet so heavily on quickly while interest rates rise? Inflation may not be intended, but it is not impossible.