Wondering what your thoughts are on the following idea? Could the death rate for such a scenario be reduced enough to prevent it being a mass death scenario?
I've got a Space Filling Republic in my setting called the Enochian Republic. The entire focus of the story was how unstable it was and those stuck within the nation as it inevitably disintegrated. The nation started as a highly prosperous, more stable and smaller entity that got lucky (or unlucky depending on how you look at it). It capitalized on global instability, destructive disasters on a global scale and the break up of other entities to grab land. Its leaders at the time were rather over ambitious and ignored the advice of their ministers, incorporating all of that land directly into the nation rather than turning them into vassal states or installing friendly administrations.
After a short period the republic was in dire straits. Its infrastructure was groaning under the strain of its enormous size and the state was beginning to collapse under its own weight. The republic, once seen as liberators from the empires which came before had yet to grant voting rights to its new citizens resulting in only a tiny fraction of the population having a say in government. The nation relied on the Auxilla system where troops were raised from one occupied region and assigned to a different region. Despite this revolts became common and the government started to slip towards bankruptcy fighting it's own subjects.
Exarchs administered huge tracts of land, manpower and wealth and many operated outside the control of the democratic government. Corruption eroded the government which redirected the wealth of the nation to a tiny political elite. Eventually the Exarchs began to fight among themselves and the republic disintegrated.
The old borders took on a strange permanence in the minds of its former subjects, who began to fight to restore the old republic with their own group in charge. It was similar to how the old European Empires drew borders during their occupations which did not represent their subjects with the peoples in these new states fighting to control these new states even after their colonial masters had left.
The world became locked in a cycle called "Humanity's Lament". It started with a growth period where one group or a coalition unified the Enochian Republic. Its enormous size and industrial capacity made it a dominant force in the world and for a limited period it became integral to the world economy and global stability. Then came the collapse period where the nation collapsed under its own weight, taking the global economy and political stability with it. The world plunged into a period of warfare and famine until the next growth phase began. To a single person within one period everything might have looked hopeless but every time a cycle completed the republic came back smaller and weaker.
In universe this led to several theories. One was that when the Enochian Republic could no longer reconstitute the world would lapse into a prolonged dark age. Another was that it would cause life for humanity to improve significantly. The last theory was that another Space Filling Empire or Republic would take its place and continue the cycle.