it is not their purpose nor their business.
Annihilus seeks to devour ALL worlds, including Ardaeon. Eventually everyone will get involved in this one way or another.
Would you mind if the Dark One and Annihilus join forces with each other? Annihilus can claim that he will devour all worlds to make a new "giant world", in which the Dark One can rule in.
The High Ones will preserve Ardaeon, but they will not seek to actively fight him; that is the concern of those spirits whom did not enter Ardaeon or any world.
His ego would probably not allow him to accept any assistance from anyone, as he perfectly believes that he is capable of accomplishing anything on his own, and not without reason. He was once the most powerful of the High Ones, after all.
The Grey Wanderer will most likely be ordered to return immediately to assist Valerian's response.
Can you delay this for as long as possible? Our own worlds will get involved in this story sooner or later, but now is too soon. We barely know each other in the game.
If the Grey Wanderer actually has a good reason to stay in the World of Elements after his task has been completed, then Lindalael will advise the Council of Doom to delay the summoning. However, even Lindalael must accept that the delaying period can be no longer than a few more days at most. The Dark One is marching across Fastidor in the meantime.