I have tremendous sympathy for the empathic pain they had by seeing those levels of exploitation. Das Kapital's first volume is basically 50% accounts of child labor and exploitation. But they had no way to know that when the state holds the capital the results are even worse, had marx seen the soviet union at its prime he would have probably revised his own theory.
Marx's reserve army doesn't hold water. And he himself denounces that one of the most common ways of the exploitative model was precisely how much the aristocrats ignored private property rights and pushed people to extreme poverty and marginalization.
He had and still has many interesting ideas, but he was very much wrong about many of them. Which is fine, all these economists got things super wrong.
And another interesting point Marx made: Look how well America is doing! Europe should be like that.