Friedman was a charlatan and monetarism was trivially debunked when he came up with it. The 60 years since have not been any more kind to it. Don't confuse stocks with flows. The whole idea of money supply as a useful metric needs to be put to bed; one would have thought the 13 years post GFC would have made that apparent.
(Of course, the Austrians have a peculiar notion that inflation can by definition only be considered as such if it's associated with increased money supply. Another reason to ignore them completely)
Between 2008 and 2020 M2 pretty much doubled and prices barely moved anywhere. This was intentional policy for the purpose of increasing inflation which stayed doggedly around zero. What does this tell us? Pretty much nothing because money supply is a useless measure.
What would happen if we halved it? Dunno, but to achieve that, stuff has to happen, and depending on that stuff, you might get price changes. I expect if the gov engaged in QT to achieve it, very little would happen.
And yet the price of a cabbage stayed exactly the same. Oh, we also had close to zero interest rates, might that have something to do with peoples' appetite for higher mortgages and shares over bonds?
But that doesn't suggest that the critical variable is money supply. It just suggests there's a correlation. There are plenty of ways of increasing the money supply that does nothing at all. Pursuade the fed to give me a trillion trillion dollars and I'll show you by not spending it (though I might spend a few mil for keeping it safe, but that's not going to do much).
The point is that lots of monetary operations are consumption neutral, including buying bonds. The only thing that actually matters from a price perspective is when a real resource gets consumed. People having more money can push the price of real goods up for sure, but the marginal propensity to consume by individuals goes down with higher relative wealth.
There's another very important effect which is the spending on things that matter induces taxation (employment taxes to deliver the thing, sales taxes etc). This is a geometric series reducing the money supply on every transaction of real goods. What matters is the flow, not the stock.
