The problem is that Marx was operating under a demonstrably incorrect theory of the source of value: a labor theory of value.

I could spend 12 hours every day making bouquets of blades of grass, but this is useless to society because that I labored doesn't matter; what matters is that the marginal value of bouquets of grass over grass on a lawn is approximately zero. I have put nothing into society.

(Edit: people have complained, correctly, that this doesn’t quite capture the use-value framework that Marx uses. But we can rescue the example simply by observing that grass bouquets do have very rare use-value — on movie sets and for weddings themed a particular way, for instance — so it is not the case that grass bouquets have no use value at all ever. It’s just that I probably can’t find a use for my bouquets, and if I can it’s probably not a very important use, so I’ve provided not nothing but very little for society. See the comments: I have replied in full there, explaining why my criticism, in slightly elaborated form, still carries its full weight.)

So, then, how do you compute what one has put into society? Why, that's exactly what Capitalism is brilliant at. And you do this and you find--actually--the amount of value people contribute is radically different. So different that you need a social safety net to take care of the individuals who are neutral or a drain, hoping to help them become a benefit, or at least because our compassion for each other extends farther than a bare calculus of what is being put into society.

The tremendous wrongness of the conception of value lies behind most of the irrelevancy of Marxist thought. When you build a system on top of such flawed premises, it is hard to recover much of use, save perhaps some inspiration and interesting questions.

And some of the inspiration turns out not to be very inspired. The described natural move towards socialized production has turned out to be historically wrong, for instance. Look at SpaceX--spaceflight has been de-socialized. Illumina--genome sequencing has been de-socialized. In many places in the United States, key government functions are outsourced to private companies (sometimes with good results, sometimes with disastrous ones--but it happens nonetheless). Innovation is rapid, and generally starts de-socialized. Powerful forces push both ways.

It is hilarious that you choose Stalin of all people to source a quote about scientific laws: "this does not mean that man can “abolish” or “form” scientific laws. On the contrary, it only means that man can discover laws, get to know them and master them". Stalin's embrace of Lysenkoism was because it was ideologically agreeable, in contrast to all scientific evidence, and caused one of the worst and most enduring blunders of intellectual and scientific pursuit in Russia. (Wasn't good for farming productivity, either.) Neither hypocrisy nor barbarism from an individual invalidates their arguments--but really, couldn't you have chosen an example where we have some shred of confidence that the speaker meaningfully believed what they said? If Stalin is as right about economic laws as about genetic ones, this is about as dire a condemnation of Marxist-inspired thought as is possible.

So you can try to rescue the scraps of Marxism out of historical fondness, I suppose. And you can certainly find endless quantities of bad arguments against Marxism--simple catchy slogans have a grip on our minds and thereby prevent most of us from actually grasping good arguments most of the time because the intellectual candy of memes is too sweet to resist despite having no intellectual-nutritional value.

But it would be more fruitful to divorce oneself from the whole thing, salvage the few insights worth saving without being burdened by the problems, and build economic and political systems that have better practical and theoretical foundations than were conceived of a century ago. The West has done this, though it does struggle with moving past the defects of unbridled Capitalism; and China has also, at least starting with Deng Xiaoping Theory--though China has an impending reckoning with the inherent contradiction between the liberating diversity of choice one gets from a robust industrialized economy, and the strikingly limited choice offered in political leadership.

Just as the United States suffers from an overly strong attachment to problematic Capitalist ideals (e.g. interpreting money as speech, essentially allowing oligarchs to dominate politics by their wealth dominating attention in elections; or too often interpreting healthcare as a luxury not a right), so too, I think, would China suffer from overly strong adherence to problematic Communist ideas (even the already-modified ones it has). The day of reckoning is likely farther for China than for the U.S., because a rapidly expanding economy is a salve that heals many wounds--but eventually the salve will run low, as one cannot expand rapidly forever.