I know it's an odd question but hear me out. Khorne is all about physical skill at arms and brute power, meaning he disdains sorcery. Nurgle is about stagnation and enduring that stagnation. So why do either one want the physical world destroyed. If chaos wins, that means Khorne no longer has warriors improving themselves, no more hatred, anger etc to fuel him. In fact, if lessor demons are technically fragments of their liege, technically that means no more blood flowing. Similarly, the change chaos brings to the mortal realm would undermine Nurgle and his powers, so why do either of them want the destruction of the Mortal Empires?