I feel people are misunderstanding Aesir here.....
Yes, the situation is not ideal, and faeries and......
But.... his ..sentiments are with humans, and honestly... the only way to resolve the current situation would be to engage in an act of pure genocide. By sinking/quick-dunking the entire island. And yes, he can......
The fruit will aid the fairies to an extent, but they'll either die out or find a way to co-exist with his favourite race, the humans. Something which isn't happening elsewhere.
This is a very subtle hint that he can afford to check up on this after a century or two to see how things are ...developing...
And nuke the place if it didn't work out...
Eh.
Nah.
There are intermediate levels to this.
Aesir's already injected some "Western" tech into "not-Japan", which might help them.
He could make that effort a bit more guided, or actually "actually" make an
effort to make a low-key systemic/cultural change.
Also, he could of course go up, check on things, find another unreasonable boss oni, and do like that original dojo woman, to make them back off even more?
Or even some up-in-their-faces mayhem to scare them off. The power balance would shift even if the oni only back off a bit, since the half-oni aren't as long-lived, so he'd reduce the available oni military force that way (even if they didn't fight, they'd fall off in numbers, by dying of old age).
But yeah, Aesir has so far not been very up-in-anybody's-face if he could avoid it.
It's interesting from a perspective point of view.
I'm still surprised that he didn't even do any closer research on the oni.
One possible story could have been to have him introduce horses to the island. Or gunpowder (imagine the tengoku bombing the oni troops?). But yeah, that's the kind of thing we'd see in less complex/thought through shonen (or even seinen) manga.
That said, this arc was unusually shallow, at least at first glance.
Or. Let's say fragmentary?