Good read. It is easier to imagine the end of the world (space aliens blow us up with the negative space wedgie gun) than it is to imagine the end of capitalism
I think they can imagine non-imperial societies; the argument is more that non-imperial societies will be destroyed by imperial ones, so you either compete as an imperial state or become part of someone else's empire. Same applies to the dark forest theory.
This is different from capitalist realism, which:
Views AES states as simply another kind of capitalism ("state capitalism")
Views pre-capitalist economies as, you guessed it, pretty much capitalism (often the markets = capitalism oversimplification, as well as ignoring the communal aspects of societies)
Predominantly assumes capitalism as an economic system in fiction
I think that’s still pretty much “imperialist realism” though, because there’s no actual good reason a communist society couldn’t compete with an imperialist one on a galactic level that I can think of off the top of my head
You're right, they'd just flatten any state that effectively resists imperialism into another kind of empire, like they'll call the Soviet Union an empire.