This will make better sense if you treat money as a trading product with supply and demand. Negative yield on an investment means the banks and investors belie

This will make better sense if you treat money as a trading product with supply and demand. Negative yield on an investment means the banks and investors believe the amount of money will shrink in 30 years due to less demand in the future or too much supply right now. They further believe that the yield while negative is still better than the amount of money shrinkage down the line. Thus a negative yield investment is still a sound investment.