Both the prime material plane and others contain many powerful characters. Even within the adventurers guild there exist S-ranked adventurers of all alignments with access to high tier magic and other abilities appropriate to their class who would be willing to accept potentially world altering quests for an appropriate amount of coin. These S-ranked heros could effortlessly resolve A-ranked quests if necessary. If the actions of any organization, god, or individual significantly alters the balance of power in this world then one or several characters with the power to do so will deal with the disturbance themselves. For example, if the player character fails to stop an orc army from invading a nearby village or chooses not to accept the quest then either some other adventuring party will accept the quest or the countries government may send knights to restore order.
For an example on a larger scale, if by the players failure or refusal to prevent Demons from invading the prime material plane leads to a demon lord gaining significant influence in a particular region other major powers will take notice. Since demons hate everyone including each other, we can expect other demon lords to take notice and send their own forces to try to claim the territory for themselves. Meanwhile, it is the sworn duty of the devils to seek out and destroy demons wherever they can be found, so eventually an archdevil will order their forces to destroy the invading demon army. Furthermore, the celestials also despise demons and will send their own forces to oppose such an invasion. Celestials dont approve of Devils interfering in the prime material plane either, so they would likely take action to ensure that the devils promptly leave after the demons have been dealt with.
In short, most plot lines will somehow resolve themselves even without player action.