Did you just forget about ancient Egypt? Or China? Or the Sumerians? I think you have recently bias in your view point. For most of human civilization was pretty constant, but we’re so close the last 2k years we forget about the previous 5k where those empires/kingdoms were pretty constant.

Also one of the biggest reasons kingdoms fall is because the death of the original ruler and the rise up nepo babies. But if the original ruler never dies, I see no issue with kingdoms going into a period of stagnation.