Is there any positive impacts if The Great Barrier Reef dies?

Assuming that evolution has happened and that our fossil record is valid then reefs have come and gone.

Animals and plants have come and gone.

Ice ages, mountains, seas, volcanoes etc have all come and gone.

It is neither positive nor negative. It is a sign of a healthy and adapting environment.

If the temperatures increased then it should cause new parts of the reef to develop in cooler waters.

There are remnants of reef even in Tasmania. A sign that once temperatures were higher and that reefs existed in different locations.

Now if there was more carbon dioxide in the air once, and a higher temperature with more rain and the world was thriving at that time what makes us think that THIS environment is somehow the correct one?

not at all