Chaos had lost their bid to win the HH right out with Hours as their checkmate, so with their hope in Horus killing the Emperor at the battle of Terra, and winning once and for all Chaos lost the HH. However....Chaos is playing the long game and is solwly gaining the upper hand, the Imperium has been living on borrowed time ever since. Time in meaningless in the warp but in realspace the decay of the imperium as of a result of the emperor being mortally wounded, and placed upon the golden throne could be considered a win for the forces of chaos in the long term. The throne is failing, the necrons are waking up, the Tyranids are approaching in greater numbers.
Chaos could just wait for the Nids to finish off theIimperium and then make their move, however this is a gamble, the Nids winning could severely weaken the chaos gods as trillions of potential worshipers would be killed by the nids, or empower them as the fighting could empower Korn and the other gods as the blood flows, the pleasure of killing or victory cointunes and the desire for hope and change is held by trillions, and the belief that the end is nigh swells the chaos gods powers, to such a degree they could defeat the nids and successfully weaken the necrons.
But the end is the beginning is the end for Chaos, for should they finnaly win their quest, the galaxy will be destroyed by the warp engulfing the matereal realm the gods will be fighting themselves for the scraps of souls that remain and in the end when all life ends the chaos gods will slowly die. And the other Tyranid fleets will seek other feeding grounds in another galaxy.