A very well-written piece!
I also think that Starcraft 1 was as 90's edgy as it gets (Not to mention the music, Terran 1 is one of the music tracks of the 90's period), but the refusal to continue this direction and instead go for a more mainstream, not quite rated Mature storyline ruined things in the end.
I think that World of Warcraft has changed them forever. Blizzard no longer is a small, geeky and wild company, but a big influence for people all over the world. They couldn't pull-off another cynical storyline in any of their franchises even if they wanted to (except for Diablo I suppose).
What's more, it's hard to write a sequel story for a game that was more or less finished. Duran and the hybrid were the only unexplored plot elements from SC1, so it became the overarching plot for SC2.
A new start was needed, and by that I mean a considerable time gap.
Look at Star Wars: The Force Awakens and how it taking place 30 years after Return of the Jedi allowed for some form of nostalgia, yet still gave new characters (and thus new plots) a chance to develop.
Compare this to Starcraft II, which took place a mere four years after the first game. Too much conflict between old and new.
Last but not least, Blizzard has never been good at telling a very detailed story. They've always been a gameplay-first company.
In most of their games you have some mission briefings and some dialogue during missions, and that's about it. But Wings of Liberty had a proper storymode (Hyperion).
Don't get me wrong, I like the chatter with all the NPCs, but it made players think that the story had to be taken more seriously. The story is considered more integral to the game experience.
Compare this to Starcraft 1 in which you could just skip all the briefings and start the mission immediately. You could pay attention to the story, but it wasn't forced upon the players.
Players had to spend more time thinking about the plot, which is good if the plot is good, and bad if it isn't. It either paid off big time for people who like the story, or further increases the frustration with the bad writing.