If USD became worthless, people would stop denominating the value of bitcoin in USD. This (or less drastic but still major fluctuations of USD) would complicate

If USD became worthless, people would stop denominating the value of bitcoin in USD. This (or less drastic but still major fluctuations of USD) would complicate questions about how bitcoin's value had changed over time, but I'm not seeing how it would create ambiguity over whether bitcoin had 'gone to zero' or not.