The Egyptian Empire, the Greek Empire, the Roman Empire, the Aztec Empire, the Ottoman Empire, the English Empire, they all collapsed. They didn't just disappear.
Your facts are a little off; so is the notion that Empires 'decline'.
Usually there are external reasons empires disappear, often the result of conquest, sometimes as the result of loss of population due to plagues and famines that cause financial upheaval, other times due to exhaustion from civil wars. And even this is an oversimplification. The reasons are much more complex. Geopolitics is a factor; it's not something that suddenly sprang up brand-new in the 20th C, even if the phrase did.
But let's talk about that idea of around 250 years for a moment.
The Egyptian empire was in existence as a single continuous culture and polity from at least 3500 BC, and powerful from around 2600 BC until the conquests of Alexander the Great in the 330s BC. So around 3200 years total. It was actually conquered by a foreign power at the tail end of the Middle Kingdom, but that only lasted until the New Kingdom around 1550 BC, so it still had another 1200 or so years to be powerful.
After Alexander the Ptolemies kept an Egyptian empire ruled by Macedonians until Clopatra bet on Antony, who was the wrong horse. She should have picked Octavian (Augustus).
The Roman Empire in the West was subjected to constant pressure from Germanic tribes, who themselves were pressured by the expansion of folks like the Huns, the Goths, the Vandals (who also took the handle!) and other groups. But even then, it started with the conquest of other Italian city-states in the 300s BC, and lasted until 476 AD, so about what, 7-800 years?
However, a significant cause of weakness in the Western Roman Empire was the crisis of the 3rd Century --there was a significant plague that is estimated to have killed millions of Romans, called the Cyprian Plague.
This plague caused tremendous losses in the population and the soldiery, caused manpower and food shortages, financial depression, and probably contributed to a series of civil wars that followed it. For example, Alexandria's population was cut to less than half, and that is thought to be typical around the empire. We know about Alexandria from contemporary sources, but other sources are not as well known. There was less production, less food, and so much loss of revenue that the coinage was debased in order to pay the bills.
This came not all that long after the Antonine Plague that killed 5-10 million people - a significant percentage of the population, estimated to be one in ten or more. It's hard to maintain an army and an economy to pay the army when that happens.
Despite all this, the Eastern Roman Empire (later called the Byzantine Empire but they still called themselves Romans) lasted until its conquest by the Turks in 1453 A.D., so lasted continuously from Constantine in the 300s for another 1200 years, for 1,000 of those years as the most powerful state in Eastern Europe and the Middle East. And that's not counting the 300 years the territory was under Western Roman control as provinces.
It should be noted that under Justinian in the 500s AD, the Eastern Roman Empire re-conquered most of Italy, Egypt, and the Middle East. Once again, a plague interrupted this expansion, called Justinian's Plague. This plague was probably the first
yersinia pestis (bubonic) plague, and killed 40% of the population of Constantinople and the Mediterranean. That's a gigantic loss of people. There weren't farmers to work the land, there wasn't trade. It's a miracle that the Eastern Romans survived as a polity for 1000 years more. But they did.
The Ottoman Empire lasted from well before the conquest of Constantinople until after World War One, so for about 500 years. World War One was the cataclysm that bankrupted it, as well as Great Britain, who couldn't even repay their wartime debts, and then had to face WW2, a struggle that lasted 6 years, and sapped much of the remaining resources of the empire, not to mention manpower losses.
There were also tremendous pressures from national groups to have countries that reflected their languages, cultures, etc., and many wars of liberation that ate up resources and patience. This in fact also disrupted the Austro-Hungarian Empire significantly, and weakened it. However, one must remember that the Austro-Hungarian Empire was the offspring of the Holy Roman Empire that started with the coronation of Charlemagne in 800 AD and lasted until the end of WWI. What's that, about 1,118 years?
In other words, these things don't happen because people and soldiery are spending their time jerking off and having orgies instead of working hard. They happen for a variety of reasons, some of which can't be predicted or controlled. Others happen as the result of bad choices - for example, the Ottomans siding with the Central Powers in WWI, and facing their own liberation movements, among them the Arabs, Yugoslavs, Rumanians, Bulgarians, Greeks, etc.
Too many people have heard of Gibbons' reasons for the supposed fall of the Roman Empire. But recall that Gibbons wrote in the 1780s and 1790s, and had no access to many of the written and archaeological resources we have today. His theories are now widely discredited.
How many thousands of years was a Chinese empire in existence? How about Japan?
Food for thought, at the very least.
And hey, let's not forget about the Empire in Star Wars that seemingly goes on and on and on...
