Brad DeLong smugly notes that Engels was wrong about the progression of democracy and the imminent failure of capitalism. Even assuming DeLong is correct (he's an honest person and a better scholar than me; I'm willing to take him at his word), so what?
Everybody who's ever had a great idea has been wrong — egregiously, totally, often unjustifiably wrong — about something. Newton missed the wave theory of light; Darwin missed Mendellian genetics; Einstein didn't buy quantum mechanics. Freud was pretty much wrong about everything, but psychology is still a science.
DeLong might have a point to the extent that some present-day communists and socialists fetishize and treat 19th and early- or mid-20th century writers if not as actual prophets then as authoritative. Anyone who studies science, however, knows that you have to try a lot of bad ideas to find the good idea — you have to kiss a lot of frogs to find a prince — and there's no way of telling beforehand what will work out in reality and what won't.
I'm a poor scholar, and I cannot support my opinion with chapter and verse. But there's a "spirit" that runs through Marx, Engels, Lenin, Mao and many other anti-capitalist writers of the 19th and 20th century, a spirit that seems supported and substantiated by the evidence we see today.
All economic, political and social relations are material relations; they are not ideal relations. It is fruitless to search for abstract social, political and economic relations that are in any sense true independent of the specifics of material reality. This point is born out by general philosophical investigation: we simply have no epistemic basis for abstract ideals.
The specific institutional characteristics of capitalism as a set of economic relations made capitalism well-suited (to a degree) to the physical economic realities in the 18th - 20th centuries. Those same characteristics make it poorly suited to the physical economic realities of the 21st century.
That we are presently in a profound economic depression that is uncontroversially caused by capitalist institutions. We knew beforehand which characteristics of which institutions would cause this depression and we knew how to prevent it. And today we know how to correct it, but we are refusing to take the necessary steps.
We know that for decades the many academic economists have not merely been mistaken but actively self-deluded. We know that for decades our "democratic" political process — elected and appointed political offices, the civil service, the "independent" judiciary and the press — has become corrupted and barely even retains the appearance of popular representation or the public good. We know that despite being far and away the wealthiest and most productive nation in the world, the vast majority of Americans are the least well-educated, least scientifically literate, have the poorest health and health care and are the most politically and socially deluded of the citizenry of any modern industrialized nation.
The "spirit" of Marx and Engels holds — regardless of their specific mistakes — that capitalism cannot be reformed from "within" by using the social and economic institutions adapted to capitalism. There is no better test of this hypothesis than the sincere and temporarily successful attempt to reform capitalism from within with the American New Deal and European income-redistributing socialism. If capitalism could be reformed from within, it's difficult to imagine two more comprehensive reforms.
And yet both reforms are in danger of actually failing; the European reforms are proving a little more resilient than the American, but when Greece blows up Europe is in for another profound hit. The reforms are failing not because they were insufficient, not because of unanticipated circumstances. They are failing precisely because the capitalist ruling class has turned every effort at its disposal towards subverting and overturning the reforms, precisely because they do in fact work, because they do in fact transfer some political and economic power away from the capitalist class to the working class.
We know that mere "regulation" cannot check the excesses, inefficiency and evils of capitalism. We know that one way or another we must socialize the ownership of capital. We know that one way or another we must radically transform our political and social relations.
And that is the fundamental spirit of Marx and Engels, as true today as it was in the 1840s or 1890s. The capitalist institutions exist not to assure the well-being of all humanity, but quelle suprise to maintain and preserve capitalism and the privilege and power of the capitalist ruling class. We might not know what alternative institutions to put in place — in much the same sense that Darwin did not know the actual mechanics of heredity, and his speculations were almost entirely mistaken — but we do know that the capitalist way is nearing its end.
To steal from Lincoln: "The dogmas of the quiet past, are inadequate to the stormy present. The occasion is piled high with difficulty, and we must rise -- with the occasion. As our case is new, so we must think anew, and act anew. We must disenthrall ourselves, and then we shall save our country."