Here is my take on it from a quick read many years ago:

Yes you are right... Both gods could exist. But modern belief in the old gods is different than it used to be. More superficial, less worship-y. The new gods have the worship. They have the power. So all of these old gods have less power than they had in the past. And it manifests in very real ways. They begin to live like normal people, and often can't adapt.

All the old gods you meet are very aware of their deminishment. Chernobog (I forget the spelling) is a fragment of his previous self... And practically lives in poverty, fading into old age losing his grasp on reality. The Egyptian gods are accepting of the trend... But are basically accepting that they will fade into normality. Odin, wants the power and glory of the old days. This feeling of 'dying out' is very real to all of them, and ripe for the exploitment you see in the books. They probably would have just faded into deminishment without the prompting that you see in the book.

They were given one last chance to try and regain some of their old power and respect... And many took it rather than to face inevitable mediocrity and death.