There are many things about which reasonable people can disagree, and interest rates are a splendid example. One reason is that there are almost no simple, clear-cut answers on account of the complex interrelationships involved. Indeed, monetary policy is only one of many factors influencing the supply of and demand for money and loans.
Other longstanding factors have also been contributing to falling real interest 1/5  BIS central bankers' speeches  rates. For example, productivity growth in Germany has been slowing since as far back as the 1970s. There are many explanations for this, not least the ageing population.