Not all, but only the demon king, as it's their ticket back to earth. But now I don't see it happening at all. It's like the point I said, whether the author will pull through till the end or not with this kind of idea.
You're forgetting a few things. Of the three who were summoned, two of them (Usato and the girl) have already expressed that they're not sure if they even
want to go back home. Usato has his family, but he was just kind of coasting through life where now he feels he has a purpose and the girl always felt like an outsider on Earth anyway and finally feels free. So, even if they lose their ticket back to Earth, they probably won't care too much. The other boy, I don't think ever expressly stated he wants to stay, but he's making connections with people and there's no guarantee that he won't be able to return via some other method.
Additionally, just because a conflict doesn't end in genocide, doesn't mean the author failed to write a good plot. In fact, the realization that the demons are just another race, and that Felm wasn't an exceptional case, opens up the moral battle of "these are just people like me, they're not fighting us because they hate us. They're fighting us to protect what's behind them."
Finally, we've already seen that the author
will kill off people on both sides, so death is still a threat.
