Fiat dies in fire. That’s what happens.
Not sure it is that cut and dry but thank you for an actual answer to the actual question.
Nothing is cut and dry but that was the best thing I came up with. My thought is that if .gov taxes fiat conversions (forex) but not spending BTC on goods and services then many (and then many more) would gravitate to a circular BTC economy. The less fiat gets used, the more it dies. Certainly wouldn't happen overnight but there is always hope.