its been two months sense the monsters from the underground have been freed to the upperworld, everyone was happy, except for sans, he had nightmares and feared that he would wake up to be in snowdin again, that you would reset the game and only he would remember the past timeline.
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".