There is no way a 40K series would be popular among normies. The humans aren't relatable because everyone is terrible all the time and much of the imagery, depsite the grimdark styling, is utterly cartoonish. The first time someone in power armor takes off their helmet and you see this little head buried in a tank of armor, people are going to laugh. No one but 8 year olds will think it's cool, everyone will just be embarrassed.