- The Orks launched a Waagh straight into the Eye of Terror, and managed to kill a load of daemons, it's true. Except none of those daemons have remained 'dead', and the Orks themselves have all died several hundred times over only to be brought back so the daemons can kill them all over again.
- The Crone Worlds are still in the middle of the Eye of Terror. They are not safe, not even slightly. The Phoenix Lords may have retaken them, this does not mean that Chaos is giving them up. And the daemons have effectively infinite forces.
- The 13th Company are also all quite possibly mad. And they've slaughtered mortal followers of Chaos, but that really doesn't achieve anything.
- The Tau, the Hrud and the Orks are all resistant to Chaos Corruption. They are not immune, and nor do any of them have any better defenses against the non-corruptive dangers of the Warp.
- The Laughing God might have a plan, it's impossible to really tell, because he's not the most forthcoming fellow. The OldCron plan would have defeated the efforts of the Chaos Gods in the mortal realm at the small cost of the souls of everyone who ever lived.
- The Emperor lost. And even if he is a Warp God now (and I kinda believe he is, to be honest), that doesn't help much. Warp Gods are shaped by the beliefs of their followers, and the God-Emperor of the Imperial Creed is not exactly benevolent...
Chaos can be thwarted. It can be denied. It can be guarded against. But it cannot ever, by it's nature, be defeated or slain. The Chaos Gods are not distinct individuals sitting in their palaces plotting evil schemes. They are the dreams and emotions of every sentient being, the fundamental forces of the cosmos, the quasi-sentient embodiments of an entire universe of raw energy. The laws of physics and sanity do not apply to Chaos, and their armies are quite literally without number. Sure, it's possible to destroy a daemon (albeit absurdly difficult rather than merely banishing them), but every time a child has a nightmare, or a man covets his superior's wealth, or a young woman falls in love, another daemon is created.
They don't call it the Great Enemy for nothing...