The heritage of the science fiction story - in the tradition of Asimov, Campbell, Harrison, etc - is based on the 'great leap forward'. Imagine one technological shift or one burst of genius, and then wildly extrapolate the results.
Asimov's robots are a perfect example - one massive assumption (innovation in robotics - BOOM!) and then thousands of pages of follow-through. Asimov's work on robots deserves special note because he didn't stop at the 'ain't it cool' stage, but also diligently worked out the social, political and personal ramifications of the new technology.
Another good example - albeit on a smaller scale - is Stephen King's "The Jaunt". This short story is about the invention of teleportation technology (BOOM!). It details the moment of genius, the immediate roll-out and the social ramifications of instantaneous travel.
This thinking dominated science fiction for decades (and still exists) - the science fiction world is composed of genius innovation and the social repercussions of its existence.
The grimpunk differs from traditional science fiction at this, the most fundamental level. In a grimpunk future, the world is projected forward, but without the initial evolution in technology.
There are two different, equally depressing, possible reasons:
1) The innovation never happened (and never will). Scientifically, what we've got now is all we'll ever get, except maybe some minor tweaking. Fundamental shifts in technology (cold fusion, FTL travel, nanotech, cybernetics, artificial intelligence) aren't happening. The spaceships of the future are the spaceships we have already, just with more mileage on them.
This is fundamentally hopeless - as it implies humanity has reached the end of its capability to grow. There are no further transformational flashes of genius.
2) The innovation happened, but the system crushed it. Grimpunk worlds invariably reference huge bureaucracies (government and corporate) with well-entrenched positions and selfish motivations to quash progress. FTL drives are sitting in a crate somewhere. The funding got pulled at the last minute. The oil companies wouldn't let it happen. The scientist with the genius intellect was pulled off the project used to make new potato chip tubes instead.
This is a different sort of despair. Individual humans still have the capacity for creative thought... it just doesn't matter.
