It may seem counterproductive to critique marx’s theories, but it has nothing to do with marx: it is an issue to do with theories in general. The successful theories we see in physics and its related subjects can be misleading:
that ‘easy’ success doesn’t follow into the larger sphere of subjects, from history to psychology, to sociology. Why do these subjects remain anemic? We can answer a question with another question: why is the subject of ‘evolution’ so intractable and why have so many brave academic minds fallen into darwinian fantasy? Dunno, but natural selection theory is a clear case of a failed theory.
The issue of history stumbles into the terrain of free will debates. And here marx, not alone, was adamant that productive force theory should proceed without a shred of such notions. And the result is a failure. The free agency of men is a crucial factor in the dynamics of history, and any theory of history must take that into account.
The eonic effect is paired with the eonic model, which you might take as a theory, still one more, but it has a catch that saves it: it may show the elements of proto-theory, but it never closes any case and so its ‘model’ factor, which incorporates free agency, survives as an incomplete not therefore a theory. The eonic effect, a term one may eschew, is nonetheless a well-documented phenomenon in world history. Experts of all sorts will try to ignore it, but so much the worse for those experts.
Adapting the core of marx’s thinking to the eonic effect is a five minute exercise, and in general the ‘effect’ remains an unsolved mystery, but one that suggests what a real theory of history/evolution would require, and that task is awesome, needless to say, incomplete.