Prices could slump as a major buyer steps to the sidelines, and for the minimal returns available in bonds more than a few years in duration, the riskof a years-long bull run reversing is too substantial for Fuss to flirt with.
And this is even more true of short-term instruments such as the overnight Fed Funds rate than it is of a longer term bond rate, because as a general rule, the shorter the duration, the lower the risk.