Gods aren’t infallible. In the AU, without the Faith of Humans, they will waste away (as they already have). It’s the very reason why Asriel turned to stardust–because the humans no longer had any hope in a world they viewed as being abandoned by the gods. It’s the reason why the gods were desperate enough to be turned into Amalgamates in the Era of Corruption.
The Balance itself is important because without it–everything will just get thrown into chaos, and no one god has the power to put it back into stability. There are too many gods existing, too many lives, too many inconsistencies and too many of those that the gods have no power over (the Unfaithful) to“force” a balance onto the world. Humans, after all, aren’t of the control of gods. They have their own Free Will, their own agent, and without their acceptance, Gods would be powerless.
It’s why Chara is so powerful and can enter the realm of gods and murder them. Gods have no power over the unfaithful.
*Reaper heard that Asriel died from illness but he wasn't aware of that--I don't think any of them are aware otherwise Asgore wouldn't have ordered Reaper to take out Chara if she fell ill..all the events in ReaperTale wouldn't have happened--*