That is already true, and with the means of transfers available (cash, gold, banking system, etc...) public institutions still give the public some confidence in that they manage to tax everyone, and pursue those who evade.

If a currency system was available that made taxation appear entirely optional to the public the trust in the tax system would erode in a hurry (Why should I pay when my neighbor or Acme inc. isn't paying?). Tax morale is alreay too low in many countries. Making more loopholes available would just not be good.