When Marx lived in 19th century England, he saw first hand the effects of laissez faire capitalism: workers working themselves to death, workers dying by the score in industrial accidents, children working and dying in mines, etc.
He predicted with certainty that this trend would continue and lead to a revolution of the many (workers) against the few (capitalists), after which the world would become a classless society free from oppression and exploitation.
Except that never happened.
Workers gained many rights, like limits on work-hours, social insurance, free education for their children (who were banned from working dangerous jobs). Most governments creates successful interventionist policy that "de-fanged" the worst parts of capitalism. I would venture a guess that 19th century workers would kill to live or at least send their children into the 21st century. I presume even terrible jobs today would look positively heavenly for someone who was forced to inhale coal dust and destroy his body for 18 hours a day, 7 days a week.
Marx's "theory" was proven wrong by the events that took place since his times. Nothing he predicted came true. This even became apparent during the 19th century, when pro-worker reforms were being introduced in Britain, which caused Engels, Marx's sponsor, to first explain this away by pointing out that Britain's riches originate from its colonies, making Britain "the burguoise of the world", and then blame Britain itself for not going along with his and Marx's grand theory: "It seems that this most bourgeois of all nations wants to bring matters to such a pass as to have a bourgeois aristocracy and a bourgeois proletariat side by side with the bourgeoisie."
(I can almost hear him getting more and more upset at the unruly masses who, instead of misery, revolution, and utopia, chose reforms and democracy).
It must be said, at least, that Marx was coming from a place of good. His heart went out to the suffering masses he lived amongst. His writings are full of compassion. But we must be fair and accept that he was wrong in his theories and predictions.
Now, because I know this will come up, as it's a popular doomer trope: there is suffering in the world. Children sew tshirts in 3rd world sweatshops. Factory workers jump from roofs working for Apple, the world's most valuable company. Numberless people live lives of quiet desperation. Without a doubt, all of that is true.
But, comparing the world of Mar'x lifetime and the world of today, it would take some hardcore rhetorical maneuvering to ignore the fact that more people than ever before live comfortable, safe lives with plentiful opportunities for self-actualization.
This can be interpreted in many ways. For our discussion, I want it to serve as evidence that we shouldn't give Marx's writing more credibility than they deserve.
Personally, I chose to interpret it in a way that means that we're simply not done yet. We've made great progress. But our work is far from done--there are still many humans out there that enjoy the most meager fruits of this progress. This must be remedied.