Here's the thing about extrapolating trends: it requires that things are in the future- as they were in the past. In short time periods (1 year, 5 years, etc) things generally change slowly enough that the trendline often holds (i.e., the near term future usually looks a lot like the recent past). But as you start talking about things like the timeline of an empire/civilization (measured in decades and hundreds of years) things tend to be less stable and more chaotic rendering that trendline useless.