As for whether Marx was "discredited" in his own lifetime, I don't know where people get that idea. I hear it or something like it all the time. Like him or not, he's a hugely influential thinker even today. So are most of his critics. It's hardly a settled issue.

But the idea of MoP isn't even part of the controversial parts of Marx. It's just a description about how part of capitalism works, as he saw it. The ideas he draws on in that analysis come largely from Ricardo and Smith, hardly "discredited" radicals.