Say BTC does gain mainstream acceptance, and your girlfriend starts buying her $18 dollar lattes in starbucks using bitcoin. They have to charge sales tax on that transaction. There is a finite number of bitcoins. If they hoard them and spend fiat, on a long enough timeline, all the bitcoins will end up in hands of the tax collectors. what then?
Governments don't hoard money. Anyway, the amount of bitcoins available would never go to 0 in your scenario because the value of a bitcoin would rise and the cost (and tax) would drop.