It doesn't need to be a unidirectional process - things get worse, the proletariat revolts, and they get better. Then things get worse again, and they revolt again. Eventually we reach a stable point where things are good enough for the proletariat to stop revolting - and that would be the communist society.

That said, while I largely subscribe to a Marxist understanding of history, I don't personally find historical determinism very convincing. Marx' theory might even have been a self-defeating prophesy, as it taught the ruling classes the mechanisms of history it needed to defeat in order to stay in power. The American right wing might not be fans of Marx, but they for sure ripped a page from his book to understand the importance of union busting.