Board Thread:General Discussion/@comment-31619784-20170328162322/@comment-31147727-20180129190736

I don't think Chara was destroying the world in that one attack: the game goes into windowed mode and the window itself shakes from her attack before crashing. I think it's supposed to be more like she severed your connection to the world (unless you choose ERASE, in which case Chara didn't destroy the world; you did) once you and your god-like power over the game world has been cut off from the game, and Flowey being dead, Chara now is the most determined in the underground, giving her your powers, like Flowey used to have your power before you started playing.

also, a bit of fridge horror here: you can't really visit the surface in the game, only in the true pacifist cutscene and credit sequence. Flowey admits that the world is a game, and once you get an ending you are satisfied with, you win the game, and the game is over when you win in. This means that as soon as the credits are finished, the monsters are just stuck in whatever area they were in last, unable to move forward, because there is no real surface for them to move forward too.