There must be some macroeconomic equilibrating force out there we don't know about. A dark force. It's sometimes slow to work, but it does work eventually. And whatever it is, it must make monetarism truer than it theoretically would otherwise be. A monetarist force. And whatever it is, it must be much stronger than a weak force like the Pigou effect. A strong force. There must be a strong dark monetarist force out there somewhere, otherwise the world wouldn't look like it does. Its existence is an empirical claim, strongly supported by observation. But what is it?

Draw an Aggregate Demand curve and an Aggregate Supply curve for a monetary economy. You can put the price level on the vertical axis, or you can put the inflation rate on the vertical axis. (Better yet, draw a 3-dimensional diagram so you have the price level on one axis, the inflation rate on the second, and output on the third.) Does the AD curve slope the right way(s) so the economy eventually self-equilibrates? Or will it spiral into ever-increasing excess demand or excess supply? That depends on the monetary policy being followed, because the slope of the AD curve depends on what you assume the central bank holds constant as you move along it.

For any vaguely reasonable macroeconomic model of a monetary economy, you can think up some monetary policy under which that model economy self-equilibrates reasonably well and reasonably quickly. And you can think up some other monetary policies under which that model economy will not self-equilibrate at all and will spiral either up or down to complete destruction.

Historically there has been a very wide variety of monetary policies followed, including some very stupid ones. And for much of monetary history, you can't even talk of "monetary policy" at all because that's a modern concept that presumes some sort of policymaker. "Monetary policy" might simply mean "whatever it is that determines how many shells people find on the beach and use as money". For nearly all of monetary history, there was no wise and benevolent monetary policymaker with a roughly correct economic model who followed a monetary policy designed to ensure reasonable equilibration. The argument from design fails, because for most of history there was no designer, and even when there was, not often a wise and benevolent one with a reasonably correct economic model. That is an empirical fact.

There are two ways a model of a monetary economy can spiral to destruction: it can spiral up into hyperinflationary excess demand where nominal GDP goes towards infinity; or it can spiral down into hyperdeflationary excess supply where nominal GDP goes towards zero.

Historically, we do sometimes observe hyperinflationary excess aggregate demand where a monetary economy spirals up and goes supernova. Sometimes these are so bad the economy has to start again from scratch, with a new money.

Historically, we do not seem to observe hyperdeflationary excess aggregate supply where a monetary economy spirals down and goes into a black hole. We do observe very bad depressions with excess supply, but never so bad that the nominal value of monetary exchange approaches zero and the economy has to start again from scratch. And we do observe economies eventually recover, more or less, even from very bad depressions.

Those are empirical facts.

That asymmetry between observing monetary supernova and not observing monetary black holes is a puzzle. There has to be a dark force out there somewhere that prevents black holes.

If monetarism were at least roughly true, that puzzle of asymmetry would be resolved. Because when we observe monetary economies going into hyperinflationary supernova, we always observe the money supply getting very large too. And we never in fact observe the money supply getting very small to the same extent to which we sometimes observe it getting very large. The biggest money supply expansions are much much bigger than the biggest money supply contractions. If monetarism were at least roughly true, and if we sometimes observed
the money supply getting very large, but never observed the money supply
getting very small, we would predict we would sometimes observe
hyperinflationary supernova but never observe hyperdeflationary black
holes. Monetarism, at least roughly, fits the empirical facts.

But what equilibrating force would make monetarism, at least roughly, empirically true? Why don't economies sometimes spiral up into infinity even if the money supply doesn't? Why don't economies sometimes spiral down to zero even if the money supply doesn't? There has to be a monetarist force out there somewhere that makes monetarism at least roughly true.

The Pigou effect is a monetarist force, and it's an equilibrating force. But it's far too weak a force to do the job. (The Pigou effect say that if there is excess aggregate demand/supply the price level will rise/fall, which will reduce/increase net wealth from holding outside money, which will reduce/increase aggregate demand. But back of the envelope calculations say this effect is far too weak to do the job, because outside money is too small a fraction of total wealth.)

The Keynes effect is a monetarist force, and it's an equilibrating force, and it's a strong force. But the Keynes effect turns weak eventually when the demand for money becomes perfectly interest-elastic. (The Keynes effect says that if there is excess aggregate demand/supply the price level will rise/fall, which will reduce/increase the real value of a given stock of money, which will increase/reduce interest rates, which will reduce/increase aggregate demand.)

And then there are strong disequilibrating forces like the Fisher effect that work against the Keynes effect. (The Fisher effect says that if there is excess aggregate demand/supply the rate of inflation will rise/fall, which will cause expected inflation to rise/fall, which will cause reduce/increase real interest rates for any given nominal interest rate, which will increase/reduce aggregate demand.)

The Pigou effect is too weak to be the monetarist dark force. The Keynes effect can't be relied on, and is offset by the Fisher effect. And then there may be other disequilibrating forces like debt-deflation too.

Empirically we observe that if you don't let the money supply explode to infinity the economy won't explode into hyperinflation either; and if you don't let the money supply implode to zero the economy won't implode to zero either. Empirically we know that monetarism is at least roughly true. But theoretically we don't know why it is true. We don't understand the strong dark force that makes monetarism at least roughly true.

Or maybe, just maybe, it's only hard to understand the strong monetarist dark force if you try to see it in a non-monetarist theoretical framework. Like ISLM, for example, where money only affects AD via its effect on interest rates. Or a neo-Wicksellian/"New Keynesian" framework, which doesn't appear to have money at all. Maybe the liquidity preference theory of the rate of interest, according to which the rate of interest is (at least proximately) determined in "the money market", by the demand and supply of money, is theoretically incoherent, because there is no such thing as "the money market". Because every market is a money market in a monetary exchange economy. And so an "excess supply of money" does not just mean an "excess demand for bonds".

Or you could stick by your theoretical framework, and ignore the empirical facts. And the historical falsity of the "keynesian" argument from design is one of those empirical facts.