We try to maintain a steady but low level of inflation because economies require expansion of the money supply to keep expanding.

Think back to when we used gold standard and the only amount of money that could circulate in an economy was finite because it was limited to the amount of gold the country had stockpiled. This was true in the late 1800s and early 1900s which gave rise to the 'Free Silver' movement made most famous by William Jennings Bryant's Cross of Gold Speech at the Democratic convention which ended up garnering him several subsequent nominations for President (but no presidency).

In a nutshell, the nation had a finite amount of cash due to link to finite amount of gold. The Free Silver movement wanted silver to ALSO be a backer of currency which if implemented would immediately see an increase in the money supply. Without a monetary expansion, means to expand the economy especially in the midwest, were suppressed. The farmers and average guy believed rightly that the financial elites who made money from money didn't want expanded money supply because it would reduce their influence on the economy and impact their gilded age wealth. You hit on this later in your piece discussing debt and interest rates.

The eventual abandonment of the gold standard allowed currency in circulation. However it was not managed well in the early 70s leading to record double digit inflation here in the USA. Nixon, upon abandoning the gold standard famously said "we are all Keynesians now" - an incorrect statement since Keynesianism as an economic theory was as tied to the gold standard as was classic economic theory.

Classic economics made sense until it didn't- with the Great Depression.

Keynesian economics that followed made sense (and somewhat still does) until it doesn't fully.

The answer to some of your question is Modern Monetary Theory which in my view DOES answer the newest nature of the economy.

But I will look into the Fisher equation more at a later time.