technically corpses can be fertilizer i guess.
Not corpses, souls/wills.
Like how they explained that the land will absorb 'willpower' back from the person who used magic (i.e. the sister's memories) back in ch 13.
Either the humans don't actually know their magic is doing that (again, the sister didn't know what her magic was really doing) or the higher ups knew and chose to use the war for more souls to sacrifice.
But that'd leave behind sediments which will spawn demons because human obviously haven't done purification rites.
I expect part of the future plot is that the West will soon suffer from massive demon outbreak because Sera's expedition, per this chapter's lore bit, has been the longest lasting one.
When they go to do their ritual to use the souls of the dead, there's too many and bam, demon horde.
Sera will then ends up fulfilling both of her choices by saving the West from demon outbreak + teaching them how to maintain the land properly, and get to marry Vuer either after returning to the East because there's more exciting things here (dragons to fight) or they become the new King/Queen of the West to ensure humans don't repeat the same mistakes.