People cannot freely migrate between the new republics due to linguistic and ethnic barriers as well as political borders.
There were no or very few linguistic issues to begin with, and people could migrate easily, for a while, until the internal and external tensions grew, which happened in most cases within a few years. But yes, the experiment is not as clean as one would hope for. Still, being able to experimentally test political and social ideas is a luxury, and this is one of them.
people could migrate easily, for a while,
There was almost no private housing in the soviet cities, and to move to another city you needed government authorization. After the collapse of the USSR there was a huge unemployment, and finding a job in the new place was nearly impossible. Ethnic tensions were also extremely high.
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?