I feel Ragnarok is an inevitable event. It can be delayed, but not prevented. And it should not be prevented.
Ragnarok might mark the end of the world. But I believe (and it is mentioned in some interpretations) that it marks a fresh beginning of a new world.
Hence, when the world is in a bad situation, a Ragnarok can end everyone's misery and the world can start fresh, on a clean slate.