every empire will face stagnation, look at America today. When the first civilizations where build half of the earth was still under ice or just been defrosted. The first civilizations grew stagnate, suddenly new threats appears, new threats brings new tactics, cant teach a old dog new tricks, first empire gets destroyed and a new empire rises to fill the vacuum.
Same happened with the Romans, Rome fell to the Germanic tribes due to the hedonistic degeneration of the late Romans, the aristocratic elite was to busy raping little boys to actual rule the territories and quell the rampant rebellions. Rome become bloated, stagnate, lions ruled by dogs. (this is highly simplified and dont add the other groups raiding Roman territory at the time, i can write about all that but you better read a book about the subject. Main point Rome became degen former enemies develop, old empire fall new empire rises)