This rule is often levelled at 40K to disparage the setting, and to dismiss some of the more outlandish and non-conforming sci-fi concepts as simply 'rule of cool'. I understand the concept of 'rule of cool' and 'grimdark' but I do not think they should be seen as used to defining 40K in it's entirety. It think these expressions are 'summaries of the superficial', sound bites to broadly describe a feeling or ethos of a setting but not really descriptive of what that setting it.
I tend to think 40K is much deeper than the sound bites, and that often the rule of cool can be explained to be reasonable and make sense with a little imagination and logic. It's not hard to come up with reason why the 'rule of cool' concept which seems air light and fluffy sweet as cotton candy could actually be the tip of the iceberg, and a great segue into a deeper background. Indeed I used this principle in my development of 'Philverse' version of 40K.
I would also note that GW has always encouraged fans to come up with their own background for their armies, and therefore the background is quite open and adaptable and seems to be able to handle almost any sci-fi or fantasy concept you care to throw at it. It seems to many ways to follow the British tradition of sci-fi such as Doctor Who, Sapphire and Steel, and even Red Dwarf. Often they are far less concerned with the technical side of things, and often mock technobabble, yet they may tackle complex issues of what happens in a technology actually existed and how humans would deal with it. It's less about the tech and more about the human drama, comedy and curiosity.
If the rule of cool is used as a segue to deeper veins of creative thought, the sound bite attention grabber to guide the reader to deeper riches, I have no problem with it.
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