The implication then is that we're in a bond bubble. When you're buying something that you know has negative fundamental returns on the assumption that someone will buy it from you at a higher price, that's the definition of a bubble.

And like many bubbles, it's entirely possible they'll be right in the short term, but it's basically guaranteed that they'll be wrong in the long term. You know exactly what a bond will be worth in 30 years, and with negative interest rates, you know it'll be worth less than now.