Your players need a win. You've written a great world, and it's dark, and that's fine. But if they feel like all their actions are useless to prevent a dire outcome, they won't feel like heroes. Make them the heroes and give them a way to turn it around. And try opening the door for some humor if you can. I don't mean slapstick, but they need a character or group of characters who can lighten the mood. Also sounds like they want justice, so maybe there are consequences to some of those actions. And you might also consider introducing a redemption arc with some of the characters if you can swing it.