Gurkhal said:
this is incorrect. While it is true that the imperials control space its no walkover. And to re-take the worlds lost, chaos holds all except for a single continent on cadia, new troops must be raised and fleets kept in operation. These resources exist but taking them will strip the defense against other threats and allow them to win ground. The black crusade was a disaster for the imperium, and a victory for chaos.
Well then, explain to us all where chaos was getting its ammunition. Where was the fuel for all their vehicles coming from? Who was repairing or delivering fresh vehicles when they got damaged or destroyed? Where were all the chaos forces getting their food (I don't think the Khorne berserker was putting up a farm for his lovely cultist fellows). How did the chaos forces repell or resist Imperial air and space superiority (kind of hard to amass troops when a couple of lance strikes or bombing runs can waste them all, dispersed troops are less effective troops)? How did the chaos forces gain ground in other places when they had no way off the planet they landed on (no space ships = no space travel)?
The infrastructure needed to run and up-keep an army are immense. When the Gothic fleet finally amassed and began winning (meaning destroying ships or forcing them to retreat out of the combat sector) the infrastructure of the invading forces (invading forces have to bring their own infrastructure with them, in this case on space craft) went away with it. The Imperium may not have been able to beat all the enemy forces on ground immediately, but without supply or transport the enemy wasn't going to do anything but degrade over time. The Imperials are also not about to leave their infrastructure for their enemy, a basic rule of warfare: if you're being overun or you are retreating destroy or render un-usable anything to large to carry with you which your enemies may benefit from.
You state, "taking them will strip the defense against other threats and allow them to win ground." What exactly do you think happened? The Tau were on the cusp of a major Imperial advance which would have wiped them out, it is the Tyranids and 13th Black Crusade which saved them. As the Imperium redirected many of the resources being used to counter the Tau and those they might have used to destroy them to the sectors with the more significant threats. In the absence of the continued Imperial power the Tau advanced with their 3rd phase expansion.
It seems careful and logical analysis of the situations supports what I stated is indeed the practically assured outcome. (oh boy here comes the: "its sci-fi hooptawhoop so logic doesn't matter" argument. Well to counter that, if anything can happen because of lack of logic, then there is nothing, absolutely nothing, which states your illogical assessment is more true than my logical one. As all arguments are equally valid under this assumption, there is no way to know which one truly occoured unless we bring in another method, say for instance: logic. When we do that, though, the illogical argument, which requried the illogical = just as true as logic to be true, is no longer true.) No military force in its right mind would allow enemies to remain behind their lines, they would have been excised, and very quickly at that (at least the ones on Cadia).