The key question of the real interest rate in the long term

In traditional macroeconomic theory, the real interest rate is in the long run determined by the equilibrium between savings and investment, and the capital stock is such that the marginal productivity of capital is equal to the real interest rate. When we look at the situation of OECD countries or the world: We can clearly see the consistency between the change in the real interest rate (its fall) and the savings-investment equilibrium; But we cannot see that the fall in the real interest rate has increased the capital stock or has reduced the marginal productivity of capital. This leads to an ineffective expansionary monetary policy or the emergence of additional savings. This anomaly can be explained: Either by the existence of a financing constraint that limits corporate investment; this is possible in OECD countries; Or by corporate concentration and dominant (monopoly) positions that lead to marginal capital productivity that is higher than the real interest rate; this is possible in OECD countries; Or by the rise in the risk premium linked to investment in corporate capital, which is possible in emerging countries. All three explanations may be appropriate.