Agreed about the other genres. I think The Difference Engine might be steampunk's only real claim to the suffix.

I've also seen an argument that the cyberpunk genre itself was punk because it pushed back on mainstream sci-fi of the time that had a theme of technology fixing everything, or better living through science. That cyberpunk made the argument that technology could change what we are, but it wouldn't fix who we are, and it would be mostly used to consolidate power and exploit. There's so much science fiction written that I really don't have a sense of overarching themes at that scale, but if you buy that, then you could argue that the mainstream sci-fi was kind of doing the corporations' work for them and that cyberpunk flipped the script.