>>212445807 (OP)Bad because normies will flatten and file the edges off of the setting. The Chuds have, unironically, already done this. Go back to late 90s/early 00s and no one thought the Imperials were "based heroes protecting their people". It was pretty clear that the Imperium was fundamentally corrupt, that's what made it compelling. Heroism in defiance of a fundamentally bleak universe is more intriguing than a society of Hard Men making the Hard Calls where every bad thing they do is justified somehow.
You can really track when the lunatics took over the asylum by how the Space Marines shift from being "merely" incredible super soldiers to being utterly untouchable, even getting glazed in other faction books. For example, in the original Dark Eldar codex the only time the space Marines show up it's as victims. There's a nice little piece of fluff about how one captive SM's superhuman endurance just meant he got tortured way longer than anyone else. Flash forward fifteen years, and now a nameless Space Marine Captain can give Lelith the fight of her life, and one space marine ship can devastate Commoragh.
The normies coming in will be the same shit in a different direction. Imperials and Eldar will become friends, the Tau will be made even more overtly evil, and the Inquisition will go from a slimy, sinister and powerful group to a bunch of flunkies.