Karl Marx didn’t repudiate capitalism. He thought it was a good human invention; but was destined to collapse because of the laws of History, and give way to communism.
Marxism is more the belief in a law of History (the progress toward a comunal paradise) than a law of Economics.
As the quintaesential modernism, Marxism only considered ‘mechanical’ laws. But History is the most organic (i.e. non-mechanical) phenomenon: everything is cause and effect of everything else.
This is why predictions are nonsensical. But our survival instinct makes them indispensable.