Board Thread:Questions and Answers/@comment-28109644-20160605153655/@comment-27701762-20160605191438

I'm not sure what you mean by "the humans would not know." In the ending they "appoint" an ambassador between monsters and humans (either Frisk or Papyrus), so part of the point is to open up relations between the two races.

You could be incredibly cynical and pessimistic and say that the humans start another war eventually. Or you could just leave off with things as they are (since the monsters shown in the credits seem to be living peacefully on the Surface) and say that humans are able to overcome their fear. It's an imaginary world, after all.