If I'm living under a dictatorship, at least I can try to escape and move to a better place. If a global government becomes tyrannical, where do we go?
Such an idea is centuries away in the best case scenario.
How about you stay and work towards changing the government? This is literally how every democracy has developed. It is also the reality for several Billion people today. Most can't just up and leave if they disagree with their governments. Borders are not open for most people.
Such an idea is centuries away in the best case scenario.