> The decline of the Roman Empire, depending on how you define it, played out over centuries, if not a millennium.

Sure, and before COVID-19 pandemics often took years/decades to spread around the globe. Agriculture took thousands of years to develop; iPhones were invented twenty years ago.

Things move faster in a world of eight billion people who can get anywhere on the planet in 24 hours.

Hence you can’t take the starting conditions from thousands of years ago and assume they will evolve in the same way now