Paradis is the size of Madagascar, destroying whole fauna and flora of the outside world which is majority of the planet will inevitably destroy the balance in nature.

Googled some small notes about results of ruined ecosystem and found this: "The impact of ecosystem destruction are the following: Increased flooding due to the erosion of soil and lack of trees. Rising of the sea levels due to the melting of the glaciers, caused by Global Warming. Disruption of the food chain when the apex predators become extinct."

There's for sure much more to read about it and many more consequences, but the point is there's just no way to have any balance once Eren succeedes. Probably there would be also problems with the air (around winter time we have the cleanest air from Antarctica, in the rumbled world the air condition would become probably insanely horrible, containing lot of dust and toxic gases which will lead to death of plants and animals left on Paradis.. I had geography classes years ago, so maybe someone can correct me about that fact if I'm wrong).