The problem with studies about the future is that they tend to too easily reconfirm the ideological beliefs of the writers. For example, the Romans didn't collapse because of environmental reasons. This makes them a tough jigsaw piece to squeeze into a deep environmentalist collapse puzzle. This is probably why Jared Diamond ignores them and instead focuses on small tribes in the arctic and easter island.

The only collapse theorist that I know of who's developed a collapse theory that avoids ideology is Joseph Taintor. Probably because he is not an economist but a professor of archeology.