This threat is still relevant, as many nations have not yet reached the economic or mental stage when they are no longer interested in territorial conquests.
If your political system requires no one in the world to defect, your political system is unworkable.
Naturally, political systems which require no one to defect are unworkable. But what makes you think that defection is an insolvable problem in this particular system? Just like individual people can act jointly against aggressive criminals, individual states/provinces/communities can act jointly against aggressive regimes.
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?