The cosmic war has been going on for thousands of years -- but the Legion seemed to have unbalanced the entire cosmic hierarchy. The titans and the Pantheon, of course, were annihilated by Sargeras during their confrontation -- eliminating the main representatives of Order as their primary obstacle. However, as can be seen from Aluneth and the mana elementals, they are not the only representatives of the Arcane in existence, nor did Arcane itself suddenly collapse under their defeat; the Aspects and keepers' powers and abilities remained completely intact, for example, as did the Pillars of Creation they bestowed upon them.
We understand the Burning Crusade, the Pantheon's destruction, the Scourge and the Lich King, they were all part of this complicated and elaborate secret scheme by the nathrezim to systematically undermine, cripple and destroy the other cosmic powers, including possibly the Pantheon of Death themselves -- but that does not explain why the others never at least tried to rally against the Legion.
What about the forces of the naaru and their keepers? Why didn't they intercede to aid the Pantheon or even before Sargeras turned against them -- as the Light is apparently very effective against fel magic, and the naaru do not seem to be susceptible to the Legion in the same way? What about the forces of Life and Death, and the powers of the Void? What was Elune, or the Pantheon of Death in the Shadowlands, doing as the Burning Crusade overwhelmed and wiped out millions of worlds and systems, billions upon billions of lives? Did the Arbiter and the Covenants not care at all about the Legion's actions? What were the elemental forces of the cosmos doing against them? Were they not concerned at the extinction of countless races which their powers influenced and even fought for?
And what about the void lords? They seemed to have mostly ignored the Legion entirely -- they even helped the Legion during their invasions against Azeroth, such as providing the ogres with a base in Blackrock Mountain, and the artifact weapons seem to be almost amused by their efforts (demons and dreams alike capable of corrupted; the end of all things unable to save the Legion from the Void, etc.) If they were so powerful, why not try to oppose the Legion openly? If Sargeras had sucessfully destroyed Azeroth, would it not have completely destroyed their plans? Launching several Old Gods into Argus -- or any other Legion world -- would have had devastating consequences, and we have seen their servants capable of dragging entire Legion outposts into the Nether to be permanently destroyed -- but why they did not intervene on a broader scale is unclear.
Long has this one's time been coming. A true servant of the fallen titan, he seeks the doom of all things. As if that would really save them from the Void.
Here you will see why the Legion's invasion is ultimately futile. All can be corrupted, dreams and demons alike.
The God of the Deep writhes in his prison, breaking free ever so slowly. You should hurry and defeat the fallen titan... there are greater battles yet to fight.
Does this not mean that the fel is always stronger than the arcane? That the titans are weak against the Legion? And Life is pretty all but destroyed? As Death has enveloped most of existence, rendering countless worlds uninhabitable and driving their native species to extinction, due to the Legion's genocidal actions? How does this make sense for a balanced conflict then? Haven't they already pretty much lost?
It has been implied that thousands of planets -- perhaps even the vast majority of planets -- have been destroyed by the Legion -- but details regarding how many worlds are NOT touched by their influence remains extremely vague and unclear. How many planets are there remaining? Is Azeroth basically the last surviving world capable of sapient life? For example, the Ardenweald quest mentions that the heroes of Azeroth saved all worlds -- not saved the last surviving worlds, or the last several worlds -- but this is very ambiguous.