yes but what is causing this slow, downward sloping decline?

free-trade, globalism, and "the free market". it turns out global free-trade just means you become the slave-plantation nation for your globalist, coin-clipping middlemen who owe no allegiance to any nation.

the cure is nationalism, tariffs to protect domestic industries and increase domestic wages, and import-replacements: you build industries domestically to replace what you used to import. you stop exporting natural resources and instead process them and use them in domestic industries. you export your global rivals into your subservient import plantation slaves.

like it or not, economics is warfare and we got tricked. we got tricked into being resource exporters and finished-goods importers.