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?
TO charge taxes they would have to know who is initiating the money. With mixers and partial anonymity its hard to know that, hence hard to charge taxes.
Have you heard the phrase "death, laundry, and taxes" before? There is a user on this board with a similar name. You're dreaming if you think you'll never have to pay any taxes because you used a mixer or whatever. Either mixers will be illegal, or bitcoin will be.