Of course, finishing the enmity between the three sides wouldn't put an end to wars and things like that. Even if demon beasts, humans, and Zasfins end up living together at some point, there would still be other reasons for battles. Wars regarding territories, inheritances, or simply because one side doesn't like the other. Then again, that wasn't a problem for Rean's group to resolve, nor would they ever plan to. Battles are necessary in a cultivation world, after all.
Nevertheless, today was the first step for a 'fairer' world, if one could call it that. The humans still had 999 more years of protection from the demon beasts, but if everything goes okay, such a long time won't even be necessary.
