I think if they had run away they would have turned into fiends. It seems the humans from long ago achieved all that they could, became bored with that and started tampering with their DNA which then caused them to turn into fiends. So, they learned that to keep it under control they have to do this milo ritual to keep them all from turning so they can live a basic happy life.
There are barriers for a reason. If they had run away, they would've probably turned into fiends because they would be outside of the miko's protection.(from pigglypoof)
I think if they had run away they would have turned into fiends. It seems the humans from long ago achieved all that they could, became bored with that and started tampering with their DNA which then caused them to turn into fiends. So, they learned that to keep it under control they have to do this milo ritual to keep them all from turning so they can live a basic happy life.