It mostly depends on what kind of demons are there and what kind of world are we talking about. I feel like there are two kinds of demons.
Coming from the east "oni", powerful magical humans with horns that are usually evil, but recently I saw them interpreted as savage barbarians, that can be civilized.
And the ones from the west, influenced by Christian vision of hell, a swarm of grotesque, distorted creatures coming into the world to devour everything.
In the first case it during the war for the humanity humans struggle to unify, milions die, kings surrender before superior magical power of the demon king. A World War, where the things human will do to survive, because there is no hero, will be much worse than this demon king could ever imagine. They will try everything. But still, some humans will try to serve the Demon King. Betrayals, politics. Maybe puppet kingdoms. There was a time i had an idea for a demon king that converts into main human faith, converts his demons, gets recognized by the Church, and then conquers the world.
If they win demons will be genocided. If they lose humans will become slaves and there will be a new story about Great Revolution, because of infiting between demons. Demon are demons after all. Or after some time some demons would become civilised and support humans in their rights. A modern world where Demon King won and then there happened an antiracist movement - to not be racist against humans. Human life matters etc.
In second type of demons if humans lose they will be devoured and nothing will be left, but mosters feasting on the world bones. So all things that can happen, happen during the war. Will the world unite fast enough? Will a simple soldier fight the great demons or run away. Will scientists or mages and artificers create new weapons against demons. What will humans become to survive.
I had an idea on the story with those type of demons. Gates open and our modern world is flooded with demons. We fight, adapt technology to new found magic, but there is too many of them. Mountains of bodies, nukes, supersoldiers. Then there comes an idea to create a Legion of super soldiers, descent into hell, and destroy the source or sources of demons. So the Legion descends, starting the hundreds years journey. With powerful technology and magic they fuse flesh, metal and soul. In hell souls can't go anywhere, so they catch them and revive if killed. They start to use demon magic and steel. They create "Maidens" - innocent souls that take on all of the other soldiers corruption, all of their suffering and truly die to cleanse it. The Legion becomes a machine of death that grinds through demons like really easy. They accomplish their aim and go back into the real world.
Where they find out it has fallen to other, weaker demons and got saved by "Gods" and their chosen. Human world got colonized by other races and humans are servants in their own home. And that where the story begins.
Anyway there can be many interesting stroies without heroes, but I think Heroes are simply a great simplification for the entire confict.