The further we get into this the more I suspect the goddess to be either a highly evolved human faced monster/golden-eyed monster or the demon emperor themself. She is in charge of the region that constantly 'holds back' the demon invasion that the rest of the world throws money, goods, and personnel towards. She doesn't need to conquer the world she just needs to maintain her status quo.
If she is a high level golden-eyed monster, then she can control the monsters enough to make invasion dates supported by 'intelligence' and oracle. As far as the summoned heroes are concerned, it may apply in revers that she gains experience only by killing other world beings which would give her incentive to cultivate them just far enough to be experience but not enough to actually challenge her and if they are near that, she can banish them to the ruins of disposal like the dark hero in the past.
All that supposition aside, I am more than ready for our MC to carry out anything resembling revenge against his former classmates.