It's a mistake to view Shai'tan's goals under a human perspective. Sure, we'd be bored with nothing to do, but leaving something undestroyed when he can destroy it is antithetical to Shai'tan. It's like some entity coming to Earth and saying 'I can't understand why humans would choose to breathe all the time. Inhaling and exhaling every few seconds seems like such a tedious and boring process to mindlessly maintain over an entire lifetime.'

But anyway, as long as the Creator exists, there will always be something for Shai'tan to destroy, because the Creator will always be creating stuff.

I don't believe that Shai'tan can destroy the Creator (nor do I believe that the Creator is out there creating stuff while Shai'tan is imprisoned... I suspect that the Creator had to sacrifice his own freedom in order to maintain Shai'tan's prison, perhaps becoming part of the world that holds him, so that existence is based on the status quo between the two deities).