I made no statement on causality. It's a simple statement on returns. You can't pay out high interest rates, or, like returns on anything, unless you're going to get improved productivity. If your productivity is declining (it probably is, considering demographics), you'll get fun times like negative interest rates. And dumbass speculative bubbles.

And low interest rates DEFINITELY do not cause speculative investment in R&D projects which could increase productivity. c.f. for example, the present! Compared to the high interest rate era of Bell Labs and moon shots; we have zilch for large scale investment in R&D for future productivity improvements.