Grünberg abandoned his early doubts about historical materialism.

Evidence

As I presume you know, I, too, subscribe to this view. I, too, am one of the opponents of the economic, social and legal order which has been handed down to us from history, and I, too, am one of the supporters of Marxism. A generation ago, I believed I still ought to raise doubts about the central pillar of scientific socialism, the materialist conception of history. But I have been taught my lesson by developments since that time, and have now given up my doubts. — Rolf Wiggershaus, The Frankfurt School, 92