Prediction in the realm of sociocultural evolution is impossible.
Consider entities such as the Taliban, the dictatorship of East Timor, the IRA, the Mbuti, Catholics, the United States of America and polyandrous marital institutions. Would the Taliban, the Pope or US WASP senators approve of polyandry? Would the Mbuti of the Ituri Forest condone American industrialization? Would the IRA endorse actions such as the oppression of the East Timorese? The problem with government is that it does act as a policing force. That, in and of itself, is a problem, but the greater underlying problem is the impetus for establishing said policing force. The impetus is that of enforcing adherence to a particular morality. No objective morality exists. A global government enforce a moral code? Which morality should be codified?
In the USA there remains derision pertaining to issues of abortion, stem cell research, racism, sexual orientation and the environment. As a nation, the USA cannot resolve its internal problems. On an individual level, consider the divorce rate in the USA. Individuals that may love each other cannot resolve their differences in many instances. Tolerance on individual, national and global scales does exist to the extent necessary for the existence of a global government. How could a unitary force not alienate its constituent parts?
In the matter of sociocultural evolution, a stimulus must be presented. This stimulus must be asserted by individuals highly esteemed by the populous to have hope for acceptance. Given that, the concept or thing must be evaluated and modified before assimilation into the sociocultural system is possible. Cultural homogeneity is not possible in the sense that any given concept introduce will be accepted only in specific ways. That acceptance will differ subtly from system to system. Such stimuli have different ramifications on an individual level.
In order to establish a global government, a global stimulus must present itself. That stimulus could be extra-planetary, environmental or a state of political incompatibility on a global scale. Barring such a stimulus, a sociocultural evolution resulting in the unification of all sociocultural systems should be impossible. Given the rise of such a stimulus and the assimilation of it, how does that effect the individual? The question still remains, to what morality will the policing force ascribe?
As an idealist, I truly hope that I will see the human population act in unison. Pragmatically speaking, I am doubtful about that possibility.
For anyone interested in sociocultural evolution, I recommend a book authored by Bruce Trigger. I do not remember the title, but the terms “sociocultural evolution” are in the title. It is a fascinating text.