We've seen how well that has gone with GW lately, its been a disaster and likely killed Henry Caville's Cinematic Warhammer plans.
Its a terrible approach, but you can get away with it for a time until key tipping points get reached. Then casuality breaks down so much your setting ends up incoherent, increasingly random, and it loses increasing amounts of hardcore fans, which are the engine that drives the hype.
The loose approach to canon is also more valid for a science fiction setting spanning the entire galaxy of billions of star systems and millions of inhabited worlds, where you can have entire wars involving tens of billions of people raging across hundreds of systems, and it doesn't even register as a blip on the galactic map. A good example is that the current go-to guy for WH40K big event novels is Dan Abnett, and he notes he was so green on 40K lore when he started he created an entire sector of space (the Sabbat Worlds) and set everything there so he wouldn't be stepping on the toes of anyone else. He only gradually emerged from that area and did other stuff and just recently both started and finished the 60+ novel sequence about the core events of the backstory (The Horus Heresy).
That approach is not really viable for a planet-bound setting where major things are happening dozens or hundreds of miles away from one another, not tens of thousands of light-years.
For the 40K series, it looks like they ran into a serious mismatch between Amazon and Games Workshop. Amazon have a
strict gender-blind and ethnicity-blind casting policy, it's very much built into their TV division ethos and I believe is a set part of their contract division. Apparently it can only be overridden for very serious dramas bound by historical accuracy. For adapted SF and fantasy works, it literally doesn't matter how the source material approached those issues, Amazon doesn't think it's relevant. Hence the very diverse casting for
Rings of Power and
Wheel of Time; that wasn't a problem
per se but some people objected to it being done in an unexplained manner that was internally consistent (i.e. in the
WoT novels the people of the remote, isolated and rural Two Rivers are described as being darker-skinned, say Mediterranean in appearance, causing the adopted, light-skinned main character to stand out; for the TV show they simply did blind casting resulting in a very mixed appearance of people for the village, with some people being paler than our lead and some people being very dark-skinned indeed).
For
40K the rumour - and it is unsubstantiated - is that this policy was being extended to the planned TV shows with an edict for female members of prominent, previously male-only factions in the lore (though not the reverse) and so on. It's alleged that Amazon rejected plans to include Sisters of Battle in the TV series and instead insisted that Custodes characters - included as they are Cavill's favourite faction - have female members among them (this was possibly a compromise after GW refused pointblank to include female Space Marines). GW gingerly suggested in recent new stories that there have always been female members of the organisation (one of the less-explored in fiction) but that was negatively received by the fanbase. Cavill reportedly just wanted to know why they couldn't use characters from established gender-blind factions (like the Imperial Guard) or female-only factions, which would allow representation/diversity without causing a needless controversy. That seems to have soured development of the project since Cavill, as he has said, values adherence to the source material very highly.
This is probably something that GW should have enshrined in their original contract with Amazon, and it's a bit odd they didn't, unless they realised that, given the current contraction of the market and the apparent enthusiasm of 40K fans at Amazon, no other TV channel or streaming service is likely to be interested in 40K projects, so they had to either agree or give up on the idea altogether.