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?
It's simple - no taxes should be used when something is paid by crypto.
Anyway, if it does, they (new owners) will sell it back in the market.