"The brink of poverty" is the key concept here.

Something like 60% of the US population presently has less than a thousand dollars in savings.

This is the reason behind the reason for the shrinking middle-class; if you have no savings, it's awfully hard to have any upward social mobility, but obviously it's very easy to move down.

People in the US want to point the finger anywhere but at themselves for this state of affairs, and largely the finger has been pointed at the government (and this was a big part of Trump's election).

But the truth is that these middle-class income-tier families that have no savings and live on their credit cards are mostly victims of their own lack of spending discipline. These families almost inevitably have giant flat-screen TVs, live in houses they can't really afford, buy designer clothing/shoes/furniture/etc, never fail to get the newest smartphone model, always buy brand-new cars, etc, etc, etc.

They buy all these things, and use credit, and subsequently live in endless debt, because owning material luxuries has become the new version of the 'American Dream'. People now don't objectively ask themselves: "can I really afford this; would it not be wiser to buy a used car/smaller home/non designer clothing/a smaller TV/wait a few generations to upgrade my phone/etc, and save some money? No. Instead, they gotta have it, and they gotta have it now.

You wanna know why America is in decline? Who's to blame for it? Look in the mirror before you look anywhere else, middle-class.