Local governments that fail to emulate the best examples will simply lose their population, who will vote against them with their feet.
It's a feature of bad governments that they don't allow people to leave (e.g. North Korea, DDR, etc.).
Just like natural selection promotes survival of the fittest, competition between different communities will promote policies that maximize human wellbeing.
Competition will promote policies that enable the community to survive. Maximizing human well-being is one possible strategy. Enslaving your neighbors is a great way to ensure that the drudge work gets done without inconveniencing your constituents. Even looking at evolution of species, we see a lot of different strategies employed. It's dangerous to suppose that optimizing fitness will also optimize some other desired property, unless you introduce some constraint (say, a big central government banning unsavory practices).
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 ...
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?