A few points for clarification:

-It’s a probabilistic model, so it only predicts the odd of a collapse

- Their main contribution was the creation and curation of a super detailed historical database: the Seshat. It spans almost 10000 years of human history with more than 400 polities from 30 regions around the world, using over 1,500 variables. Based on this data, Turchin & al devised the mathematical model for the prediction.

- One key area is to find surrogate data when others are not available. For ex. body size could be used to describe the nutrition and economic situation of the population.

- In 2010, Nature asked experts and super-forecasters for their prediction of 2020. Only Turchin predicted the coming collapse of America.