May 21, 2018:
The obvious problem is that we have no theory of what would come after.
We have Marx's highly rigorous theorization of the capitalist mode of production, in Capital. Some hints about cooperatives, and transcending the law of value. That really is it.
There's a democratic impulse behind this absence. Marx and Engels were determined not to construct the top-down utopias for which they criticized earlier socialist thinkers. Their democratic commitment was that working people would invent their own future.
As an aside: you'll never find a more bottom-up, anti-statist, anti-elitist stance than this. Marx was far more anti-statist than even the most consistent anarchist. This principled opposition to telling working people what to do is one dimension.
There's also the inevitable constraint that one person can't do everything, even when there are two people closely collaborating. Maybe there'd have one day been a Capital Volume Seven speculating on possible socialist modes of production, not as definitive roadmaps but perhaps as signposts suggesting paths the movements could consider. If they'd lived to be four hundred.
Where the downside is that there's a vacuum, a theoretical empty space which as we know will always be filled by the dominant ideology. So that the Stalinist statist vision of "socialism in one country" reproduced capitalism with a single state capitalist. While the reformist visions of parliamentary socialism have the effect of perpetuating a reformed and rationalized capitalism without providing a coherent roadmap to something superior.
In my opinion this is the key theoretical priority for young movement intellectuals today. Produce rigorous theories of possible socialist modes of production. Where the law of value is transcended. Let the movements choose between them.