wot u say is true but it will be a very long time before Abaddons forces are sent back, and i qoute
"The Thirteenth Black Crusade has broken the Imperium’s hold upon the Cadian Gate- perhaps forever. The raging tempest of the Eye of Terror has surged forth, engulfing those worlds lost to Chaos. The Imperium no longer bars the gate to the Eye, only a small channel remains through which Imperial Navy vessels may pass to bring aid to the desperate forces upon Cadia.
At the close of the Thirteenth Black Crusade, Cadia still stands. But she stands alone, a failing beacon flickering against the encroaching night. Total war is come to Segmentum Obscurus, and all hopes of repelling the invaders are dashed. The Imperium must now consolidate its grip upon those worlds it still holds, and prepare to fight a war that will not end within the lifetime of any of its combatants. While Cadia still stands, humanity has reason to hope, but Abaddon the Despoiler has finally achieved what he has failed to do on twelve previous occasions, over ten thousand years- he has breached the Cadian Gate, and none can now hold back the inexorable tide of Chaos unleashed upon the Imperium of Man." final Newsletter
chaos didnt win fully but they did get a victory