>but that completely misses the point of the article: that the money supply isn't large enough
It seems you misunderstood my point, which is that bitcoin is not "21 million coins". There really are many more units, with the option of increasing the divisibility past eight decimal places in the future.
You are still missing the point. It is not divisibility that makes bitcoin unsuitable as a spending currency, it is the fact that its price keeps rising. Imagine if you are presented with two coins one that historically inflates and another that deflates and are asked to spend one and hold the other. Which do you pick? An inflating coin will always replace a deflating one as a medium of exchange. This is called Gresham's law.
A coin with a fixed supply will never be suitable for daily transaction because the demand for currency isn't fixed (due to economic and population growth).
This isn't true though, the supply of the money has no relation to whether or not something becomes a store of value. The key attribute is that it's finite (deflationary).
However, I need to say that a finite currency isn't deflationary forever, it can only be deflationary until there is no more demand for the currency, which has to happen at some point, then the currency becomes stable and mature.
The money supply is important as well, bitcoin is infinitely divisible (easy network upgrades possibly required) so for all practical reasons money supply (liquidity) will never be a problem for bitcoin, but it has been a problem for every single currency which came before. Gold especially because it's expensive to deal with changing and re-issuing the money supply each time you want to add liquidity.
Today, it's much easier with digital banking but liquidity is still not 100% all the time so when liquidity needs to be added or removed, it's done in unnatural ways, which leads to instability in markets.