Now, it's acknowledged often enough that while capitalist societies have some diversity of power, even when they become oligarchies, socialist and communist societies always seem to revert to kingdoms.

Is this a coincidence? I don't know much about Marx, but he has obvious dislike to urban bourgeoisie, that was so common among landed aristocracy. Engels opposed land taxes as an "utopist" idea. The only goal of socialism was to be the nationalisation of industrial capital, without any idea how housing and resources would be allocated afterwards in "the dream society." When asked, he said "socialism is not supposed to be fair, it is supposed to be effective."