It is probably the influence of F.R. Leavis, but I find a bit of a gap between the only true conservative, Socrates, and Marx in this tradition, but I suppose it is difficult to be too critical of Mill, so he'll do to fill it, fair play.
While Marx's economic predictions floundered, and his thoughts on how socialism was actually supposed to function were cursory at best, there is no question that scholars consider him to be one of the most influential and consequential moral philosophers and social scientists of the 19th century.
The transformation of the guilded age of robber barons into liberal social democracies is in no small part due to reactions to Marx's critique of the dark sides of capitalism and keen insights into the human condition.
It cannot be overlooked that totalitarian communists coopted Marx's name, although it is clear Marx never supported or advocated for a totalitarian state based on terror and oppression