Well, there’s a great deal to be said about ‘whinging about capitalism’, frankly.
Karl Marx wrote a great deal more than The Communist Manifesto, though I’d bet not one person in a thousand could tell me what any of those other writings are (and, to be honest, I think fewer than one in a hundred could tell me even THAT he wrote The Manifesto).
For quite a bit of his life and writings, Marx focused on the downstream effects of Smith’s Invisible Hand analogy. Marx showed how such a large-scale system could easily brush aside the poor and powerless. He believed deeply that such power could be countered through collective action. Well and good, and not something that could be easily disputed, honestly.
Where Marx is failed, however, is in both his followers and his opponents. During the 20th century (and to a certain extent the 19th) those who read and agreed with Marx - after they achieved power - proceeded to scare the living hell out of everybody by becoming tyrannical overlords such as Stalin and Mao (as well as other, smaller players on the world stage) who advocate revolution worldwide, regardless of the cost in human lives and goals.
On the other hand, Marx’s opponents have spent more than 100 years demonizing him and his ideas for their own agendas. By using Marxism as a whipping boy - aiding by Marx’s followers in re: scaring the hell out of everyone - various political and philosophical leaders have been able to secure their own positions. It has - to a certain extent - done no little harm to see unfettered capitalism run rampant than it has done no little harm to see unfettered communism run rampant.
For me? I’d say that Marx would have been horrified to see the excesses done in his name and angered to see the excesses done in opposition to his ideas. Because, in the end, that’s all they were. Marx believed that large organizations - governments and corporations - would oppress the poor and working class if the organizations could get away with it. This is, to me, indisputable. However, Marx failed to really see how a middle path could be charted that would allow for economic growth and the increase of average wealth while still affording protection for all through a strong safety net.
This is, after all, a debate still ongoing. Though to my thinking, it’s one that is trending towards a middle ground (thank goodness) where some form of European social democracy will eventually become the norm in western societies.
But to say that Marx had little impact on the world following his writings? That’s silly. He wasn’t alone in his positions - he was a part of a larger debate and philosophical movement during his time writing in Europe - but for better or worse people know him and at least one of his writings (though again, I’d best most haven’t read it). It’s not too strong to say that most of the western political movements on the 20th century: The rise of Americanism, the New Deal and the Great Society, social democracy, Stalinism, Maoism and even Nazism, developed as a response to or in opposition to people who read and followed or rejected Marx.