For example, Marx's theory of the profit rate points us in the direction of understanding the mechanisms by which constant capital costs can be reduced and the rate of surplus value increased.
Marx's theory is surely both incomplete and wrong in some of its theses (e.g., the necessary commodity character of money, perhaps the deskilling thesis, the elasticity of the real wage, perhaps the strength of the profit rate to equalize, perhaps on how limited capital saving innovations would prove to be, the persistence of or reasons for absolute rent and of course the effects of the credit mechanism), but the point is that the theory does allow itself to be empirically invalidated and is thus not a pseudo explanation; Marx certainly did not anticipate all the dynamic results of capitalism, but this does not make his theory a pseudo explanation or justify a post Marxist approach. Especially since there is no comparable empirically testable theory which is of the equal calibre (neo classical economics? Parsonian structural functionalism?). So yes we must go beyond the incomplete Marx; his explanation is incomplete but one is best guided by Marx's theory in how to complete and revise that theory.
I would suggest that in the name of post Marxism we do not smuggle in a rejection of explanatory social science as such for an only hermeneutic or interpretive social science, an exclusively emic rather than also etic enterprise, a purely subjective rather than also objective social science.
Rakesh