The "national" level will always matter. It's a problem of governing and risk management. Different regions have different problems, and the bigger an organisational structure, the more complicated. The hierarchical system with different levels of legislation and regulation at least works.
In some sense we are already working towards a global government by claiming human rights to be universal. Out of that endeavour there will (in the far future) emerge a central authority or assembly which effectively enforces human rights and free trade.