Pilots fear the Death Spiral. If the plane is already too slow, pulling back on the controls in an attempt to gain/hold altitude further slows the wing speed and increases the stall.
As so-called “investors” flock to so-called “safe-haven” US debt, the otherwise productive capital or consumptive capital sits idle. In addition, the bond holder actually loses money depending on inflation, and takes on duration risk.
So, fear of extreme debt leads to slow growth, which leads to flight-to-safety (no pun intended), which leads to lower yields and even slower growth.
In both cases above, we know how it can end.