Regarding flat ontologies, I agree that it is likely that a post-Marxist would experience some modification of theory and method by addressing these perspectives (and there are a wide range of flat ontologies from OOO to Latour and DeLanda). I would note though that a flat ontology would not argue that hierarchies do not develop or exist. Instead, they only assert that no hierarchy or asymmetry is ontologically necessary. Latour’s problem with Marxism and other “sociologies of the social” as he terms them is that they presume that particular asymmetrical relations are at work in a given situation before they begin their work. That is, if ideology/power is totalizing and overdetermining in its operation, then it is always the same explanation for every situation or event. However I don’t think that post-marxist analysis needs to work in this way. One can still do systemic analysis but there’s more work to do as Latour denies the leap from a singular situation to a global explanation.