What I see happening is a multiple break-out of plague, war, famine, and economic collapse which leaves the developed world severely stricken at all levels and all within a very short space. Ironically the folks already living in areas of the world on a subsistance level may do better and have more freedoms for a time then those who are barely surviving in such places as NYC or London. Governments will become brutal and any pretences of civil rights will disappear. Religious freedoms will be curtailed accept for those “religious” movements that support a “conformist” philosophy for conformity and “cooperative team spirit” will be seen as the only way to survive the horrors that have fallen upon the world.
As for “prepper” types, who have stocked up for the coming “black swans”,the only way they’ll get to keep what they have and not have what they have stored up “confiscated” ( in the name of the people) from them is by banding together in defensible regions and/ or secreting themselves in such remote locations that it will not be worth the effort of regional rulers, who would be barely holding things together as it is, to get at such “survivalists”.
The U.S. may very well break up into regional countries unless strong force, even the threats of limited use of tactical nuclear devices, could be brought to bear against such balky populaces.(an 1860’s style civil war couldn’t happen now, a modern President Lincoln equivalent with control of limited nuclear weapons such as neutron bombs could strangle a rebellion quickly).
Given simultaneous multilevel (and geopolitical in scope) black swan events however, even forming basic limited government in many regions may be months in reconstituting. Hence the U.S.A. break up may occur anyway, simply as a means for the survival of newly disorganized and isolated people groups.
(and as the bible says...”and the living will envy the dead”)
This would be likely if mortality were to be 5%-25%. However, should mortality go much higher than that, people will be much too busy surviving for government to be effective.
90% short-term mortality would result in wars within a generation as people try to occupy the vacant spaces. 99% short-term mortality will be near 100% in some areas. 99.9% short-term mortality would leave 7 million and make it a challenge for people to find people to have children.
Many more would starve/be killed by other pathogens/animals/other people. I strongly doubt species extinction, even at 99.9% short-term mortality, unless there is a second wave of nukes/biological weapons/other assorted insanity.