I believe that someday our Society will be rich enough that all legitimate government functions can be funded entirely from voluntary contributions. There were a few Greek City States that succeeded in funding themselves by hitting up the Rich for voluntary funding. In the Old west labor and funding for a Civic building or a bridge was often a voluntary affair.
As long as those paying could direct what projects they want funded, then every project would have both popular support and support of those who pay the bill. It would sure put a limit on Government.