@Vincentius : Or the 'sacrifice one to save many' cycles.
The Demon Lord rise to take control of demons that's growing in numbers beyond control, gathering 'evil' into one spot.
Then Hero come in to tactical nuke them, thinning the herd back to 'manageable' numbers, oh and Hero also die in that fight so there's no reverse happening on 'good' side.
Or, ironically, it's the reverse.
Human start growing too large in numbers, world spawn more demons + Demon Lord to cull them down and Hero as a stop button to prevent the Demon Lord from culling too many.
Repeat when the balance start tipping again.
Which would make the two of them not doing their function a problem for the world.