Honestly, Chaos wanting to erase the entire world never made much sense for me. Sure, turning it to a Chaos Wastes-esque hellish paradise is one thing. But why would you destroy your own pantry?
And I am totally with you on this question. I would go even as far as to suggest that not even Chaos Warriors should be presented as a united front. Sure we have the Everchosen to lead them around, but who honestly thinks that the Gods or their followers would ever consider putting aside their own agenda? Archaon should be absolutely swamped by domestic issues ("Nurglites again infected the rest of the horde with a new, fatal plague." "The Slaaneshi managed to taunt the Khornates into attacking everyone in sight." "Tzeentchians won't stop betraying everyone to everyone.")