You’re serving me a softball by pointing out that this model is a drastic simplification. Those are precisely the grounds on which Keynes and other critics rejected monetarism, because of its oversimplification: “Critics of the theory argue that money velocity is not stable and, in the short-run, prices are sticky, so the direct relationship between money supply and price level does not hold.”
Now, applying my line of criticism to this model, what I’d say is that the deterministic link between money supply and the price of goods, or the claim that the “general price level of goods and services is directly proportional to the amount of money in circulation” hides the complexity of social and political arrangements. The act of hiding is implicit in the weak analogy between economics and physics, which neoclassical economists extended in their expectation that there would be just such airtight quasi-physical systems for economists to model with these mathematical “laws.”
This is wannabe Newtonian mechanics in economics. And what’s being hidden is that societies aren’t so deterministic or mechanistic. The medium is message here too.
