And the people did rise up and successfully tried to fix the problem - there was a big socialdemocratic movement that culminated between the world wars.
What he underestimated was the ingenuity with which the capitalism reinvents itself (and creates new forms of private property to gobble up - free computing in RMS's sense just one example). He also overestimated ability of most people to understand the problem (it's lot more lack of emotional rather than intellectual capacity). I would say alienation is central to Marx, unfortunately alienated people can be so indoctrinated to fail to consider the alternatives. Most people seem to prefer to suffer through hardship rather than demand an alternative solution.
That's interesting, this is a pretty generous representation of him in my opinion. Its been a while since I read some of his writings and went down the rabbit hole listening to long form interviews of historians that studied him though, my memory could he failing me!
My understanding what that Marx envisioned a future utopia and saw two revolutions, both presumably violent, as necessary to get to the end goal. At best I could see him being indifferent to the suffering and deaths required in his model, but I never got the feeling that he would regret or would want to avoid the suffering. If I'm not mistaken, one of the revolutions he expected and wanted to see happen would have leaned heavily on the poor and working class turning on the rich and powerful to the point of killing most or all of them.
Again, I hope my representation is accurate here. I don't have time to dig back in to fact check this right now, just sharing my recollection.
But I think there are people who consider forceful redistribution of ownership to be violence, even if no human is actually physically harmed in the process. I don't and I think there is a distinction to be made.
Edit: Nevermind, there's a lot of interesting debate about this on the Internet.