It is unclear why interest rates are trendingly decreasing since a long time

https://www.visualcapitalist.com/700-year-decline-of-interest-rates/?fbclid=IwAR3PXlPebpSQ_wi4FVvgEkKZviNAbQICz60EVJiY6ystERKc_9sgxxWdrqo

Debts?

Bonds

Other factors

It is true

One basic reason: there is less demand for money.
Money in the world is fiat money which value is instantaneously fixed by change markets and influenced on the medium-term by Central Banks policies.
Two causes: CB printed so much money since a long time and still more since the 2008 recession that the stock of dollars, yen and euros exceeds the needs of the economy.
The economy needs innovation for increasing productivity and it is more a problem of how to boost valuable innovation than finding money to boost the economy.

The consequence is that the mission of CBs which are presently focused on inflation, growth and jobs is probably non-sense in the setting of very low productivity gains. Whatever they do, the trend of deflation, rate of malinvestment will last.

https://www.bankofengland.co.uk/-/media/boe/files/working-paper/2020/eight-centuries-of-global-real-interest-rates-r-g-and-the-suprasecular-decline-1311-2018

https://dievolkswirtschaft.ch/fr/2017/04/kugler-05-2017fr/?fbclid=IwAR1qGmHlLJUSpnTmmCcgwF-GKfMVt6xSWEVFb8D9FmDWFce2jXIp-wT1cKk