Poetic as it is, “the history of all hitherto existing society is the history of class struggles,” is surely quite false, both as an empirical description of history and as a summary of Marx’s broader theory. For the same reason in both cases, in fact. It’s not true that, throughout history, “oppressor and oppressed, stood in constant opposition to one another,” because, as the Marx writes a few lines later, “in the earlier epochs of history, we find almost everywhere a complicated arrangement of society into various orders, a manifold gradation of social rank,” while “our epoch, the epoch of the bourgeoisie, possesses, however, this distinct feature: it has simplified class antagonisms. Society as a whole is more and more splitting up into two great hostile camps.” The direct confrontation of oppressor and oppressed is not something actually visible in history, but an underlying tendency that has yet to be fully realized. And, indeed, the way in which class struggle is not simply visible is an important feature of Marx’s theory.
I’ve complained on many occasions about people who take class to be a positive identity. The point is that class is not an empirical category, but a structuring abstraction. This is especially true of the proletariat (the most real of the real abstractions that are class, and so also the most abstract). The “propertylessness” of the proletariat is not just a lack of possessions, but a lack of positive qualities: that is what makes the proletariat abstract labor. As abstract labor is a subtraction of distinguishing qualities, so “proletarian” names an absence, the absence around which the whole capitalist system is structured.
The idea of a generic absence structuring a social system sounds a bit like Laclau’s idea of the universal as an empty signifier. The problem with Laclau, though, is that this idea of the empty universal is based purely on an abstract philosophical argument about the nature of meaning, but there’s nothing to tie this argument to actually existing class structures. What Laclau loses is Marx’s historical argument for the development of a particular form of absent universalism in the capitalist system in particular; we’re left with a formless mass of particulars. Ironically, given Laclau’s stress on the importance of the political (as opposed to the supposed “positivity of the social”), this is actually a de-politicizing move. Rejecting the Marxist account of class as an abstract structure underlying empirical social groupings leaves us with no way of evaluating the alignments and movements of these shifting empirical identities (this depoliticization is visible, I think, in Laclau’s theory of populism).