tl;dr: whales.
In “Marx on Money“, Suzanne de Brunhoff describes the theory of money that Karl Marx presents in “Capital”.
Money, for Marx, emerges in three stages prior to capitalism.
In the first stage, gold becomes a measure of the value of all other commodities rather than simply one commodity among many.
In the second stage, gold coins become the medium of circulation. Once gold becomes de-materialized in this way its role can then be occupied by fiat currency.
In the third and final stage, the emergence of hoarding paradoxically introduces money “proper”.
de Brunhoff writes that “Hoarding is a demand for money as money…”, an interruption in the circulation of money that “…serves to ceaselessly preserve and reconstitute the money form as such, whatever the deformations, transformations, and disappearances it undergoes as a result of the other two functions. Produced by these, it becomes in its turn a condition of their functioning.”
In the crypto space, hoarders are known as whales. They remove their coins from circulation with the expectation that this will ultimately provide them with more utility than immediately spending them. In this they act just like hoarders of gold coin or fiat currency. To quote Marx, “The money becomes petrified into a hoard, and the seller becomes a hoarder of money.”
Whales therefore establish that cryptocurrency is money according to Karl Marx.