Earlier quoted context omitted.
The decline of the Roman Empire, depending on how you define it, played out over centuries, if not a millennium. It is such a bad analogy for the kind of within-our-lifetimes collapse that is being speculated about here that it undermines the original concern. The decline of Germany as the leading scientific powerhouse in the first half if the 20th century would be a better case study imho.
Historians today tend to emphasize that, even if the Western Empire ended in the fifth century, ultimately everything goes back to the Crisis of the Third Century. That took place with a short enough space that it must qualify as a within-our-lifetimes collapse.
There are many episodes in Roman history that people would have experienced as decline or even collapse within their lifetimes, including multiple sackings of the city of Rome itself. But in most of those cases things actually did bounce back and the Empire continued to dominate. That's kind of the point I was making. Empires would seem to be far more resilient than contemporary observers would think if one only looked at Roman history. But the problem is that the Roman Empire is a bit of an outlier in that regard. Ancient cultures of Central America would be interesting to study there (there were far more than just the Mayas, Inkas and Aztecs, most of them relatively short-lived, hence good case studies for the question of collapse). Unfortunately much less is known about them than about Romans.