How complex systems collapse - sudden or gradual ?

Niall Ferguson argues that complex systems, when they collapse, they do so suddenly. This is slightly unintuitive, right ?
Complexity and Collapse - Niall Ferguson for ForeignAffairs.com

Excerpt - ....empires do not in fact appear, rise, reign, decline, and fall according to some recurrent and predictable life cycle. It is historians who retrospectively portray the process of imperial dissolution as slow-acting, with multiple overdetermining causes. Rather, empires behave like all complex adaptive systems. They function in apparent equilibrium for some unknowable period. And then, quite abruptly, they collapse. To return to the terminology of Thomas Cole, the painter of The Course of Empire, the shift from consummation to destruction and then to desolation is not cyclical. It is sudden.