Bond investors do not sit passively and wait until governments cheat them. One startling statistic that emerges from the UBS note is that 55% of US government debt is due to roll over during the next two years[...]. If investors think inflation is on the way, they will simply raise the nominal interest rate they desire. Worse still, they may also increase the real interest rate they demand. Higher real interest rates (which discourage business investment) are associated with lower economic growth. And slower growth of course makes it much harder to reduce debt-to-GDP ratios.
I'd love to get my hands on that report from Paul Donovan of UBS; but I don't have an account at UBS.