What got him in trouble was thinking his ideal society was inevitable.

I'd substitute "viable" or "possible" for "inevitable." But we need to cut Marx some slack. When he was writing, England and the USA were democracies, but capitalism was unconstrained. Something was missing.

That something was Fordism, the technological advance that made workers into customers. From that point on, keeping the mass production customer happy started to matter, and one way to do that was to treat that customer better on the job. Happy workers are better customers. That realization lowered the magnates' resistance to universally applicable constraints, i.e., constraints that did not harm their competitive position. Democracy as a brake on business was empowered by business finding profit in selling to voters.

It is understandable that Marx did not see that coming. Today’s Marxists don’t know that it has come.