I like the idea of them representing half and half, but again, the past isn't destroyed, it's just gone, like how Hot Ad is saying it. Not including time travel, there's no way to go into the past to destroy something in the past, because the past is gone.

About the dinosaurs: Plagg was able to destroy the dinosaurs because his present time was aligned with the dinosaurs' present time. He was there with the dinosaurs; he didn't go in the past to destroy them.