A single world government is one of the older ideas around. I thought of it a very long time ago myself. It’s a sophomoric idea that consolidators first think of. It isn’t good.
A single world government would be oppressive, repressive and monopolistic to the point of dictatorship. A hierarchy would result with ranks seeking to serve yet reach the peak of power. Like an absolute monarchy or dictatorship an error in government policy would doom everyone that felt they weren’t already doomed by the onerous decrees of the sole planetary power.
Competition in the marketplace of ideas is requisite for producing good ones. Today in the U.S. Democracy politicians already ignore good ideas and go with popular ones that ensure political support and positive reinforcement from powerful special interests. With one government ruling the world there would be no marketplace of ideas or competition with the ideas of the government. The government would be the absolute authority on everything negating the intellectual productivity of virtually everyone not high on the table of ranks.
About the only good reason for one world government is the establishment of a Military dictatorship capable of forcing everyone alive to follow a particular environmental plan to save the planet. It would be a benevolent military dictatorship as well as a very bad idea. There are many better ways to go. If one really desires a world government then wait unto the return of the Lord God and be content with nations and national independence until then.