Funny. Anwar Shaikh, who I would describe as a Marxist economist, who builds his theories in the line of Smith, Ricardo and Marx, uses in his new book "Capitalism - Competition, Conflict, Crises" the same criticism against mainstream economics and proposes against them his own theory of "real competition". I just slooooowly working myself through this book, so I can't say too much about that yet.

Anyhow, I don't think that Schumpeter is completely right. First of all, there aren't any real monopolies around, just oligopolies. And Marx had said something about them (although, obviously not enough for Schumpeter). Also, they don't mean the absence or negation of competition. I can recommend this article by Michael Roberts: https://thenextrecession.wordpress.com/2016/05/17/monopoly-or-competition-which-is-worse/.

Furthermore, there are marxist schools (the Monthly Review school) which say that capitalism has entered a new stage of monopolism and they have done a great deal of work to explain monopolies from a Marxist point of view, or rather, to implement them into Marx's theories. I mean, I doesn't agree with that school at all and think they are wrong, but it's still worth to mention.