Impact

In the novel The Lost World, Ian Malcolm puts forth a theory for why the relatively short effects of the meteor impact resulted in absolute elimination of all dinosaurs - even those that survived the firestorm and artificial winter. The ones that survived had to adapt in a short period of time and then adapt again. It was too much for them to handle two major adaptations within a few years:

...major changes push animals closer to the edge. It destabilizes all sorts of behavior. And when the environment goes back to normal, it's not really a return to normal. In evolutionary terms, it's another big change, and it's just too much to keep up with.

A related alternate theory: the effects destroyed the food webs and ecosystems. This completely changed who was best adapted to the new world.  That the environment later returned to normal is irrelevant - and not really true. While the winter may have ended, other underlying ecosystems would have been irretreviably changed.   When Apple released the iPhone, Blackberry had a lock on business and Nokia  was in the lead on handsets. Those pioneers are still around, but too much has changed for them to easily return to where they were. Their business plans can't be built around "get back to the market share we had" - too much is different.  

It may be that the Fourth is heading the same way. People made it through the difficult times, they are starting to dig themselves out. Even if nothing else was to change radically, there is going to be a period of consolidation, of figuring what's next - in some cases, of trying to go back to the way things were. Some attempts might even work.

Retirement has to be reconsidered - whether or not it's really an option. There may be thoughts of moving from a starter home to something better...but prices have been bid up by companies with capital and an interest in renting. They might want to look into a different career....but wages have gone down, and there are still plenty of the out-of-work looking.

This next adaptation may be rough.