Isnt it stated in the new csm codex that the crusades were just a gathering of resources to fight the war that would come after breaking cadia? so theyre not failures just a process
That is correct. The new codex puts much more emphasis on the Thirteenth Black Crusade by suggesting the previous twelve were waged as mere precursors in preparation.
But the thing I really like is the discussion between Talos and Abbadon in soul hunter, it basically comes down to the fact that both the empirium and the chaos SM are screwed big time, all humanity is not the just the emp lovers,
Take into account that conversation occurred before
the Thirteenth Crusade.
Foothold on cadia...can't the Imperial Navy just bomb/lance them into oblivion?
Or cant cant imperials just land some siege engine of doom and macroshell them back into the warp?
This quote from the final newsletter of the Eye of Terror Campaign answers your question:
The Thirteenth Black Crusade has broken the Imperiumís 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.
The Eye of Terror has surged forward, engulfing huge swathes of the Cadian Gate within the tides of the immaterium. The Imperial Navy cannot reach Cadia in sufficient numbers to challenge the forces of Chaos, even with the intended redeployment of "almost the entirety of Battle Fleet Gothic, along with a substantial proportion of Battle Fleet Solar."
I am not sure why the 13th black crusade is such a success for Chaos ?
The Eye of Terror campaign was a victory for the forces of Chaos; Cadia is largely in their control, as is orbit around the fortress world. Abaddon has breached the Cadian Gate and total war engulfs Segmentum Obscurus. The Imperial Navy may still be in control of the space lanes, but there seems no chance of resealing the gates at Cadia.
But the real success lies in the campaign's future potential: Abaddon's intention is to "tear open the gates of hell" and unleash the Daemonic Legions upon the Imperium. Realising that a direct assault on Terra is all but impossible with his mortal Legions, he intends to forge the "Crimson Path" in order to sustain the Daemonic hordes indefinitely and expand the boundaries of the Eye of Terror to engulf every world he conquers. Terra is still the ultimate objective of the Thirteenth Black Crusade.