A world government can only—ONLY—be established and maintained through the consistent use of brutal force.
Like a corporation that gets too big, becomes inefficient, and falls to its more nimble competitors, governments too get too big and inefficient. Unfortunately they rarely have competitors (although the Governors Association could one day become a de facto competitor to the standing Federal Government who knows.....).
A world government would have to have such thinly spread and tenuous power to have any hope of succeeding, I honestly think the only thing a theoretically successful world government could do is to provide a universal currency and smooth over free trade laws. It would be arguable whether it actually was a government or not.
Give a world government the force that all governments need to maintain power (police, military, materiel, etc.) and you have a worldwide Soviet Union or Nazi Germany. The amount of force required to keep it together would *necessitate* that much force.
Have you noticed that we are back into the ages of industrial majors? Remember when AT&T was the primary communications company in the USA and Bell Canada took care of our neighbor. There was only four or five steel companies and they owned almost all of the other smaller ones. Five major car manufacturers. A dozen major airlines and half a dozen aircraft manufacturers. Half a dozen major chemical companies who own or control all of the fertilizer trade which also own or control the major seed companies. Half a dozen meat processors who control all of the livestock feed lots who are the major buyers of all ranch raised beef. The same for fowl.
Each and every industry is reliant upon the others in some manner. Make major changes in one and you affect all of them. People are even easier to control. Cause a shortage in anything they rely on and they’ll move. The days of the self reliant family is coming to a close. Most folks don’t have any idea how to live with electricity for six months. Offer them low cost electricity and they’ll move to it.
Notice that I haven’t used force or threats to shape any part of this civilization. I haven’t given this post more than ten minutes total thought. Imagine how smoothly I could shape it with a weeks worth of notations. You could do it too if you thought about it.