While the current cycle of declining interest rates has been running for at least 25 years, the most recent iteration of the period has been exceptionally benign. Since the end of the global financial crisis, Corporate and, to a greater extent Government, borrowing costs have run at the levels close to, or even below, those observed in the 1950s-1960s.

Since 2002-2003, FFR, on average, has been below the risk premium on lending to the Government & corporates. This has changed in 4Q 2017 when Treasuries risk premium fell below the FFR and stayed there since. In simple terms, it pays to use monetary policy to leverage the economy.
Not surprisingly, the role of debt in funding economic growth has increased.

And, as the last chart below shows, the relationship between policy rates (Federal Funds Rate) and Government and Corporate debt costs has been deteriorating since the start of the Millennium, especially for Corporate debt:

In simple terms, risk premium on Corporate debt has been negatively correlated with the Federal Funds Rate (so higher policy rates imply lower risk premium on Corporate bonds) and the positive relationship between Government debt risk premium and Fed's policy rate is now at its weakest level in history (so higher policy rates are having lower impact on risk premium for Government bonds). In part, these developments reflect accumulation of Government debt on the Fed's balancesheet. In part, the glut of liquidity in the banking and financial system (leading to mis-pricing of risks on a systemic basis). And, in part, the disconnection between Corporate debt markets and the policy rates induced by the debt-financed shares buybacks and M&As, plus yield-chasing investment strategies, all of which severely discount risk premia on Corporate debt.