Marx's work predates the marginal revolution, so he can't be personally faulted for not understanding concepts like marginally decreasing utility. But at least

Marx's work predates the marginal revolution, so he can't be personally faulted for not understanding concepts like marginally decreasing utility. But at least as I understand his work, much rest on his association of labor with value. And if you want to focus only on commodities, well, this is where the effect of marginal utility is most pronounced. And since marginal utility pretty much destroys the relationship between labor and value, it should be sufficient to discredit Marx's economics. (not that this is the only argument) without the threat of communism we would be probably still forced to work without unions... I can't imagine how you built such an idea. Particularly since unions in particular are fundamentally communist.