Killing a daemon in the Warp doesn't actually do anything to it according to Draigo's fluff*.
Never said it's that easy, just that hitting the actual Warp presence is what you would need, since realspace is just a "shadow" of the Warp presence. Sure, mostly when killed in Warp they still just come together after a while, but that still better than Banishing which just limits them appearing back to when they can get next ride back.
Well, killing a demon is actually really easy. What isnt easy is to get everyone to go along with it
The Chaos Gods are reflections of the general mindsets of the material peoples. If the races of the galaxy would stop being so damn Grimdark, the Chaos Gods would follow suit. However, Humanity (And the Eldar, to a certain extent) instead of focusing on un-fucking up their society, focus instead on "Beating" Chaos. This leads to them making more and more fanatical and stupid, grimdark decisions that make their own societies even more fucked up, which in turn causes Chaos to be even more fucked up, which, in turn, due to its effect on everyones psyches, causes everyone else to become more fucked up. Remember, when the Old Ones were still kicking around, the Warp was the "Realm of Souls" and wasnt all that bad. It was mostly just an interdimensional plane full of weird spirit energy and shit. However, when the Old Ones got slaughtered ruthlessly, they're race-wide turmoil messed up the Warp. When the Ctan started eating souls, it caused the Warp to be even more messed up, as suddenly its main composite was getting ripped out. AND THEN, when the Eldar Murder-fucked everything, the resulting society-and-galaxy-spanning completely messed up methodology of thinking caused the God Slaanesh to be "born" (Though really Gods dont die or become born, but thats another topic)
In short, if everyone put down their weapons and tried to be peaceful, then everything would eventually quiet down, and Daemons would either A. Die or B. Become something extremely dissimilar to the daemons we know now. But instead, Chaos causes Material turmoil, Material Turmoil disrupts Chaos, and so everyone is stuck in an eternal cycle of GrimDarkness that could be ended in literally a few years.
In Short: Wanna remove a Daemon from existance? Stop being a shitty person.
That is actually very problematic, because if you all just stop being shit, and stop shooting and in general become peaceful, the deamons aren't going to magically disappear that second. Chaos is going to go on a fucking retarded field trip through the whole Galaxy, now being unopposed, torturing, slaughtering and in general being fun as fuck. Once you start it, it's self-perpetuatng cycle. The only difference is that Khorne would become less powerful.
I heard something about a civ-style warhammer 40k 4x, have I gone mad or did it just fly over my head?
Gladius has a hexagonal grid... that's the closest I've heard to anything like that.
Yeah, Gladius. I actually did hear it described as 4x by devs, which probably means less building a civ, and more building military bases and getting reinforcements instead of technology.