The three generations thing is probably more of an observation over time and definitely not a golden rule. There are of course exceptions to this observation as you pointed out.

That said, as observed through history, an empire eventually falls no matter how great it was or long it was in power. The Zhou Dynasty in Ancient China lasted almost 800 years, the Roman Empire lasted over 1000 years, Ancient Greece lasted about 1200-1300 years, but eventually they all fell.