Considering negative rates in the future, you'd want to load up on long term bonds even if they yield less than short term. If you're locked in short term you'd miss the ability to get the long term and end up worse off in the long run. If in 3 years short term rates are -0.5 and long term is 0, but you locked it in now long term, you'd be 'sittin pretty (haha)' come that time and for the next 10 years.