Enslaving your neighbors is a great way to ensure that the drudge work gets done without inconveniencing your constituents.
In the preindustrial societies, slavery could certainly be profitable. In the modern economies, mechanization generally made forced labor economically inefficient. This is why most countries like DDR are no more and North Korea has become an outlier. Obviously, aggressive regimes may still prevail even against far more advanced neighbors if the latter are disunited or do not have a comparable military force.
The question is whether a big central government is the only thing that can protect us from such threats. Would not it be possible, for instance, to limit central governments only to the tasks that cannot be handled locally (e.g., military defense)?
In the modern economies, mechanization generally made forced labor economically inefficient.
Although this may become true as automation becomes able to do more things more cheaply, China seems to be a very strong counterexample at our current/recent technological level. Prison slaves, Forced student labour is central to the Chinese economic miracle, Laogai or "reform through labor" programmes, etc. America's prison labor system is also quite scary.
Historically, the evolution of government systems was mainly driven by violence, with invasions and revolutions being the principal agents of selection process. The rules of the game were predetermined by our environment - land was a limited resource, for which our ancestors had to compete, if only to ensure the survival of their descendants.
The 20th century introduced a game changer. As agricultural productivity in developed countries rose by orders of magnitude and natural population growth practically came to a halt, possessing a large territory stopped being a necessity. Countries with little arable land, ultra-high population density and no natural resources can now not only feed their population, but also achieve top living standards. These changes may open a fundamentally different route for societal evolution – one that would not be based on violence or compulsion.
A small thought experiment - imagine what would happen if central governments cede most powers to smaller territorial units:
Unfortunately, there are serious obstacles to the successful implementation of this idea:
Do you think these problems are solvable?