> One could make the argument that free marketism inevitably brings about capitalism which inevitably turns into growthism.
I wouldn't make the first part of that argument, because I'm not convinced that "free marketism" is even a real thing, much less a precursor of capitalism. It seems to be position presented as a rhetorical defense of capitalism, but doesn't seem to exist independently.
But what this article calls "growthism" is one of the core features of capitalism called out by the people who named it. Capitalism doesn't inevitably turn into growthism, it is (or, at least, includes as an essential feature) growthism.
> Much like Benjamin Tucker's argument that Marxism inevitably ends in Stalinism (he made this argument as a contemporary of Marx, so this wasn't hindsight).
Stalinism -- or, more to the point, Leninism -- requires both Marxism and an environment that is unlike the ones described by the descriptive elements of Marxism in a particular way into which one attempts to translate the prescriptive elements. Given those ingredients, I'd agree that its pretty much inevitable that something like Leninism will at least be attempted.
Marxism itself doesn't inevitably become Leninism/Stalinism, but its existence makes the existence of such a system quite likely.
> I happen to actually believe Schumpeter's model better (that capitalism turns to gas and water socialism turns to growthism).
I would argue more that the empirical evidence is that capitalism plus the existence of socialist ideas leads fairly usually (even if not inevitably) to a variety of mixed systems that feature a basically capitalist structure with socialist elements (including growthism), of which "gas and water socialism" is an example.