This is going to sound like concern-trolling, but I'd thought that Marx was afraid of the possibility of the Social Democrats winning (i.e., a modern mixed economy forming) -- since this would leave the current holders of property-qua-power still possessing it, while making the workers comfortable enough to no longer desire revolt. But my memory here's faulty, and I suspect it might have been Lenin who was hostile to the Social Democrats -- although Lenin has never struck me as a particularly subtle thinker, to say the least. Did the Social Democrats even exist as a faction in Marx's time...?
(As for the rest: let's hope that the term "modern mixed economy" continues to spread. Calling the postwar Western system "capitalism" has led to a lot of confusion.)
From what I understand, Marx was opposed to reforming capitalism, since he thought it could steer the working class, at least temporarily, from the total revolution he was pushing.
I have read that Social Democracy arose in part from the failure of the working class to revolt and overthrow capitalism. In the communist movement at the end of the 19th century there were two different responses. One, lead by Eduard Bernstein, decided instead to turn to electoral politics and try to move the economy to socialism in gradual steps, and so they joined the other groups that had formed the Socialist Democratic movement.
The other group, lead by Lenin, decided to change the party into an elite group that would lead the working classes to revolution, and this lead to the communism movement as we know it and all the totalitarian states it produced.
(As for the rest: let's hope that the term "modern mixed economy" continues to spread. Calling the postwar Western system "capitalism" has led to a lot of confusion.)