Most wealthy individuals spend a much smaller percentage of their income on consumption compared to the middle class. It then makes sense for aggregate demand to decline as the size and overall spending capacity of the middle class declines.

Unfortunately the dysfunction in U.S. politics prevents the government from pursuing policies that explicitly buttress the middle class. What happens instead are more indirect interventions like the Fed-driven stimulation of asset prices and increased defense spending. (Both of which benefit relatively narrow segments of society.)