Dynasties have a natural life span like individuals and no dynasty generally lasts beyond three generations of about 40 years each

There is a quote that periodically makes the rounds.  It is from the author G. Michael Hopf in his book, Those Who Remain.  

Hard times create strong men. Strong men create good times. Good times create weak men. And, weak men create hard times.

It certainly describes a distressing proportion of history but is counterbalanced by Adam Smith's observation that 

Be assured, my young friend, that there is a great deal of ruin in a nation.

Sometimes you can have repeated spells of bad rulers or terrible administrations and yet still pull through to some revived glory or prosperity.  And other times. . . other times, it is the last straw.  

I had not realized that Hopf's description of cyclicality was first described by Ibn Khaldun in Muqaddimah in 1377.  Ed West makes the connection.  

Ibn Khaldun's description of the cycle of history is the asabiyyah cycle.  From Wikipedia in Asabiyyah.

'Asabiyyah or 'asabiyya (Arabic: عصبيّة, 'group feeling' or 'social cohesion') is a concept of social solidarity with an emphasis on unity, group consciousness, and a sense of shared purpose and social cohesion, originally used in the context of tribalism and clanism.

Asabiyya is neither necessarily nomadic nor based on blood relations; rather, it resembles a philosophy of classical republicanism. In the modern period, it is generally analogous to solidarity. However, it is often negatively associated because it can sometimes suggest nationalism or partisanship, i.e., loyalty to one's group regardless of circumstances.

The concept was familiar in the pre-Islamic era, but became popularized in Ibn Khaldun's Muqaddimah, in which it is described as the fundamental bond of human society and the basic motive force of history, pure only in its nomadic form. Ibn Khaldun argued that asabiyya is cyclical and directly relevant to the rise and fall of civilizations: it is strongest at the start of a civilization, declines as the civilization advances, and then another more compelling asabiyyah eventually takes its place to help establish a different civilization.

 

More particularly:

Ibn Khaldun describes asabiyya as the bond of cohesion among humans in a group-forming community. The bond exists at any level of civilization, from nomadic society to states and empires. Asabiyyah is strongest in the nomadic phase, and decreases as civilization advances. As this declines, another more compelling asabiyyah may take its place; thus, civilizations rise and fall, and history describes these cycles as they play out.

Ibn Khaldun argued that some dynasty (or civilization) has within itself the plants of its own downfall. He explains that ruling houses tend to emerge on the peripheries of existing empires and use the much stronger asabiyya present in their areas to their advantage, in order to bring about a change in leadership. This implies that the new rulers are at first considered 'barbarians' in comparison to the previous ones. As they establish themselves at the center of their empire, they become increasingly lax, less coordinated, disciplined and watchful, and more concerned with maintaining their new power and lifestyle. Their asabiyya dissolves into factionalism and individualism, diminishing their capacity as a political unit. Conditions are thus created wherein a new dynasty can emerge at the periphery of their control, grow strong, and effect a change in leadership, continuing the cycle. Ibn Khaldun also further states in the Muqaddimah that "dynasties have a natural life span like individuals", and that no dynasty generally lasts beyond three generations of about 40 years each.