Modern technology makes it very possible for a single government to control the entire planet.
Given the competition between societies we can expect more capable governments to out-compete less capable ones.
Pure elite rule is the system with the best potential.
https://vintologi.com/threads/elite-rule.24/
The only limiting factor i see is that usually when a country becomes a dictatorship the ones who grab that power do not actually govern particularly well in addition to the modern notion that it would somehow be wrong to expand your borders.
It's just a question of time before a capable government emerges with the resources needed to gradually gain control over the entire planet.
It might actually be better if this happens early since competition between societies can result in many bad things such as very extensive environmental destruction, valuable citizens killed in wars, destruction of immense resources, focus on building a strong military rather than actually making the country a nice place to live in.
https://vintologi.com/threads/societal-survival-of-the-fittest.979/
Grabbing power over the entire planet would of course be extremely difficult so we can expect the final winning government to be highly competent and this will also allow them to do a good job at selecting their successors (which they may have already had to do successfully in the past).
How big chance does CCP have of grabbing control over the entire planet?
Instability in non-authoritarian systems
It's difficult to make a system that isn't authoritarian stable, having different independent branches of government would result in multiple points of failure and could result in a very destructive power-struggle such as what happened with the roman tetrarchy
We are currently seeing more and more ruthless competition for power in the US where the different factions go increasingly far to push their ideology, even schools for small children are now battlegrounds for the culture war.
This is not surprising, if you do not play dirty to remain in power someone else will. Only fools abstain from using the power they already have to secure and expand their power.
Some people have suggested trying to limit the power of government but unfortunately that cannot even be done in the first place. All you can do is change the power-distribution such as restriction the president by empowering the supreme court.
If you somehow manage to successfully restrict government power you just allow power to grow outside of the official system and if this continues the government falls and other people grab