I read this on a website exploring political theories, in particular relating to poverty:
"The various explanations of poverty can be seen more simply as two opposing perspectives:
"Explanations of poverty, which concentrate on the poor themselves. These include; dependency theory; the culture of poverty thesis and the cycle of deprivation.
"Explanations which focus on the social structure. These are Social Democratic and Marxist theories.
"Each perspective is associated with opposite ends of the political spectrum, with those on the right tending to individualise social problems, and those on the left tending to socialise them. Very different policy implications flow from each."
Marxist analysis of the origins and operation of the capitalist system is one that many political theorists, whilst disagreeing with several aspects of it, find insightful and useful. As I understand it, Marx didn't urge the overthrowing of capitalism but believed it would eventually reach crisis point and destroy itself.
Marx appears primarily to have been a theorist, interested in political structures - why they were formed, how they operate and whom they primarily benefit.
His description of what is now often referred to as "the underclass" as: "the lumpenproletariat, the dangerous class', the social scum" is one that I can't imagine John McDonnell, or most people who think of themselves as "socialists", espousing.