Tossing a huge range of different properties in a blender is fine, ripping off a single property is just lazy. Which sounds more original pure Harry Potter fan fiction or ER the TV show + Harry Potter + Cheers the TV show.
Draw evenly from M*A*S*H + Dresden Files + Mass Effect + Dexter and your going to end up in some strange and interesting places. Play through Starcraft 1+2 on the other hand and the story is fine if a bit bland.
I absolutely agree that Blizzard's worlds are weaker than Games Workshop's. GW has a real talent for genre fiction, where Blizzard struggles with the basics of plot structure.
The thing I disagree with is that Blizzard's problem is that they 'copied.' Everyone copies! The bits that Blizzard copied are the most compelling part of Starcraft. The reason it is weak is that they were not better at drawing on the ideas of others. They did not understand what was compelling about 40k, so they create this unsatisfying narrative mishmash.
Warhammer FB -> Warhammer 40k is a great example of this being done better. You could type out the same kind of demeaning summary of Warhammer 40k as you did for Starcraft: "Tyranids are gross, chitinous just like Geiger's xenomorph; the Eldar are just space-high-elves; The squats are just space dwarves." It's all true, as far as it goes. But - because GW is better than Blizzard - they went further and added other ideas (many of which also came from other works). The problem is never that they drew on other sources. In fact, looking at other worlds and using the core of what 'works' about that world before transforming it is at the center of basically all good works.
Draw evenly from M*A*S*H + Dresden Files + Mass Effect + Dexter and your going to end up in some strange and interesting places. Play through Starcraft 1+2 on the other hand and the story is fine if a bit bland.