Scale matters

As an abstraction or a concept or something, money is this superposition of a medium of exchange and a store of value.  It has value because it is exchanged; if it stops being exchanged, the value goes away.  If it doesn't have a sovereign to protect it—to guarantee a certain predictability and consistency of exchanges against all comers—the value goes away, too, quite possibly because the exchange stops.

That's money on a scale of nation-states or the meta-scale of economies where you're talking about trade networks.

Another interesting scale is the personal; money is not, to an individual, what it is to an economy.  Money to an individual is the exchange rate between life span and agency.  However much money you get for giving up some amount of time determines the agency available to you.

When you look at it that way, it's obvious that the intractable constraint is time; individual people all have about the same amount.  The upper limit on the agency you can exercise is then set by either the exchange rate you can negotiate or the amount of other people's life span you can control.

Being extremely useful to a lot of people—being able to negotiate a high exchange rate for your lifespan into agency—has inherent limits.  Most notably, luck; becoming a wildly popular artist is not something you can arrange to do merely through skill and diligence.  Luck makes things inconsistent, and people generally loathe inconsistency.  They want to be sure what they'll have tomorrow and sure what they can give their posterity.

This drive for consistency and predictability produces structural pressure toward being able to co-opt as much of other people's life spans as you possibly can, because that maximizes your agency.  The more agency you have, the more you can produce consistent outcomes for yourself. Without some powerful constraint on the amount of agency any individual can obtain through the money economy (that is, how much of other people's life spans they can take) and a non-monetary economy to provide agency to the constraint, the money economy iterates toward a condition of nearly universal de-facto slavery—where you haven't got enough agency to meaningfully refuse anything—because that's what a money economy does.  It's an agency maximizer, but it's an agency maximizer only for the luckiest descendants of previous winners.  Everybody else has theirs taken away.

This is why you want to have and enforce income and asset caps that limit people to the amount of agency a not particularly lucky person could exchange their lifespan for.  That's the minimum constraint to have a system that won't iterate into a condition of near-universal slavery.