true, you might want to look at empires that lasts for centuries after rapid expansion
alexander the great, napoleon, the japanese (1940ish) don't count because they cannot sustain the expansion
ironically, mongol empire (and its vast expansion) lasted for 3.5 centuries. there must be some other reasons that neither cruelty nor barbarism can explain (in fact, empires based on cruelty/barbarism don't last long)
The pattern I see throughout history is that large organizations endure until some disruptive technology takes them down. (Greek hoplite armies vs. light infantry and archers, armored knights vs. longbows, cavalry corps vs. tanks.)
The modern age has simply accelerated the whole process. Technological change occurs increasingly quickly now, and so these "overturnings" happen more and more often. Because large organizations take time to form, the balance of power has shifted to small entrepreneurial organizations.