Two determined spirits green and red,
Shall bring your reign to an end,
After many years of sleep,
We will all go free,
Today monsters and humans will not get along,
But the peace will come back before too long.
So, monsters and humans have been living together on the surface for quite a few years now... That's an accomplishment! No resets, no new timeline. Just a future continuing forward like everyone could have possibly hoped for.
However, not everything is hopeful. You can't expect to live a life where everything is sunshine and rainbows. Even with no longer having the threat of someone like Chara, things can still happen when you least expect them to. You can't always have your "Happy Ending".