Sure, this won't change much, but it would at least make the main evil faction in the game more dignified than ''SPACE TEAM ROCKET THAT'S TOO STUPID TO GET SHIT DONE''. Again, it makes the Emperor being so fearful of Chaos corruption look stupid if all they do is fight each other and get nothing of substance done in 10K years.
Yes, that was the point. Chaos corruption causes people to act like fools and get nothing done which was completely counter to the unification and conquering of the galaxy that the emperor wanted to do. 10000 years later, religion is back, there's a bunch of infighting, etc. the emperor worshippers might as well be chaos themselves. The unification of terra and great crusade before the heresy was all of what, 500 years? And a large part of that before even finding the primarchs? Humanity was very quickly on its way to getting its shit together right up until the emperor fucked up again, just like he had repeatedly fucked up throughout the entirety of human history.
Chaos isn't the only faction of terminally dumb characters in 40k.
Even if the necrons all woke up, they're so concerned with their own infighting most of the time they see humans as ants on their lawn.
Dark Eldar couldn't give a shit what happens to anyone so long as they can keep raiding something to keep up the torture shit to avoid falling to slaanesh.
Eldar have been written into a brick wall and make no sense, same for harlequins. There's a reason they just gave up with the ynnari.
Tau have no idea wtf is going on in the galaxy and are constantly getting kicked in the teeth by orks, nids, etc.
Orks only exist as a punching bag
Nids are an eldritch hive mind horror that can't really get its own lore because it can't have characters in any meaningful way(similar to how the borg went to shit in star trek when they needed a borg queen and crap). And can't be much more than a "looming menace" because if they did show up in full force and win, GW would just have to hit the reset button.
GSC only exist as a plot device/punching bag(like the orks) at this point.
Votann barely even have lore so don't count for anything. They've got one short story outside of their codex, and that's mostly about necrons.
If this is just becoming a realization to you about 40k, I don't know what else to tell you. But what you want is never going to happen unless GW wants an end times and I don't see that happening without fucking everything up even worse. If you want to play 40k, but want things to be more like ww1... that's exactly what the Horus Heresy game is for, and thematically it's fucking boring due to a distinct lack of variety(which is also why that legions imperialis shit was a massive disappointment instead of us getting epic 40k back).