The Gods are beings that see all of time at the same moment. Think of them like viewers of a tv series, who can fast forward or rewind at will. They can 'write' the WLW and similar interventions into the flow of history flawlessly. (No Mark on the earthquake, etc.)
The No-God is the being that ends time. He is a virus that eats away at the inside/outside connection. Static on the line. No amount of flipping through channels can ever catch sight of him, because he is the off button.
When the No-God triumphs, the Gods are blind. One day the Gods will be blind to the No-God, and therefore they ALREADY ARE. They are atemporal beings. 'Spoiled', if you will.
So for chumps like mortals, who view the series one episode at a time...this is super worrying. Imagine you are starting to read TDTCB, and you hear a bunch of readers who have already seen the end talking about how Kellhus took down Conphas. That is hugely worrying to you as a Leweth fan, because it necessarily means that Kellhus doesn't marry Leweth and live with him forever.
Similarly, if Momas isn't earthquaking the No-God it means that the No-God will one day blind him. That means that Kellhus isn't going to succeed in stopping it from being created, because if he was going to do that it would never blind Momas, which means Momas could see it now/in the past, and earthquake it.