The difficulty with this is that a model without boundaries is useless as a predictive model, and that describes all of the declinistic models proposed in the article. I believe the author would agree. And if it fails as a predictive model it is useless as a normative model. Hence to depend on any of these for reliable advice on, say, foreign policy, is to go beyond the capabilities of the model. Kennedy got his nose rubbed in that one.
Clearly many who saw decline were just wrong. I see decline of the West in the loss of confidence in the culture. For example, the civilization's people must be "arrogant" enough to force immigrants to assimilate. This is now lacking everywhere in the West.
America may recover that backbone in response to some future crisis. We hope it will not be too late.