These are again the tangential issues. I think you as well as the person I was initially responding to are conflating this issue with the one of decreasing population. I agree with you - I do not see population decline as a problem in and of itself.
The problem is population dynamics. Those that are more well to do and most capable of producing productive offspring are not doing so. By contrast those least able to care for and produce productive offspring are multiplying. Imagine we start with a nation that's incredibly prosperous - there are 9 wealthy families and one poor family. But each generation the wealthy families only have 1.6 children on average - 80% of what's necessary to maintain their population. By contrast the poor family has an average of 3 children - 150% of what's necessary to maintain their population.
Generation 1: 9 wealthy, 1 poor
Generation 2: 7.2 wealthy, 1.5 poor
Generation 3: 5.76 wealthy, 2.25 poor
Generation 4: 4.6 wealthy, 3.38 poor
Generation 5: 3.68 wealthy, 5.06 poor
Generation 6: 2.94 wealthy, 7.59 poor
Of course as has been mentioned some of the poor will become wealthy and some of the wealthy will become poor, but all things being equal even in very socially balanced nations the parents' income is strongly correlated with the child's. People of no means reproducing beyond their ability to sustain themselves is not a new problem, and is something people have pondered for centuries. But what is new here is that people of means are no longer producing enough to even sustain their population. When you combine these effects together, it turns poverty into a sort of virus that spreads and expands rapidly. Our utopia where 90% of people are wealthy (somehow.. that doesn't even make sense if you consider the connotation of wealthy, but that's another topic) ended up being a nation that was heavily impoverished in just 6 generations.
I also think a somewhat interesting pattern to observe in those numbers is that there was a population decline during periods of prosperity, but as the nation became more impoverished its population began to rapidly grow. The generation where the poor greatly outnumber the wealthy being the first generation to have a greater total population than we started with at generation 1.
The whole point here is that all the work in the world against poverty means nothing if those that escape poverty do not reproduce, while those that remain within it do!