When Shadowlands was first announced, especially with the Maw and all I was thinking about why they don't simply destroy souls that are dangerous enough to the cosmos, instead of sending them to eternal torment. You already have the punishment angle with Revendreth, so the Maw is the solution for those you simply want to get rid of, because they are too dangerous, why not kill them?
I think the answer is in Shadows Rising, in the speech Bwonsamdi gives to Talanji about things always coming back, like in a wheel of time. A concept or a powerful entity, everything always simply comes back once it's gone, even if it may take a long time (and it may not be the very same entity or thing, but the idea and the concept stay the same, so if it's that evil, it comes back as another evil, no chance of coming back as something good the next time).
I think that's the crux of it all, nothing can really get erased out of existence once it leaves an actual imprint on the cosmos. Maybe the Maw is a way to protract this process, so the evils come back much later than they would have if they simply got 'killed' (because you can't erase them permanently).