Just wondering, Emeralda put the end of the gate far into the sky so it would not be detected, but Suzuno put it on the floor and nothing happens.
With what we were told in this chapter, i cannot see a happy ending for the daemons: the closest to it i can think of is for the daemons to exhaust the magic energy and mingle with the humans, but humans are all the time in conflict, so they would get found out easily, and the energy would not be enough to fight back.
More successful would be to strike a secret deal with the human kings: we invade an area of your choice to make you look as the good guys and stop conflict amongst humans, and we get the humans to keep the fear within them with each incursion. But i doubt this would work for long.
Another would be to take some humans as slaves to the daemon world to feed from their despair, similar to Matrix, but humans would strike back sooner or later, to look good in front of their vassals.