Come to think of it the whole miasma thing might sorta balance out humans having saints.
Something like, fully qualified saints can do a lot of damage to demons, BUT they don't seem to have a good way of detecting demons AND it seems like some human settlements need a saint stationed there for protection.
Meanwhile a decently strong demon can make the surrounding area significantly more dangerous for humans just by hanging around. Seems like that could potentially lead to places that used to be fine without a saint desperately needing one.
Overall saints have a significant edge in an actual fight but demons have an easy way to make it so only a few saints can actually come to the front lines.
Side note: I like that the demons want to "reclaim" the human realm. I've thought about this before regarding similar settings where demons were apparently present in the human world at some point in the past before being driven out and have now reappeared, a human nation in that scenario wouldn't think of it as "taking over" another country, they'd think of it as reclaiming land that was previously stolen from them.