In "The City of God Against the Pagans", Books 1-13 [pgs. 665-667], Saint Augustine writes:

Who would doubt, however, that the state of things at the present time is more virtuous, now that marriage between cousins is prohibited?

Saint Augustine is a prime example of the Dunning-Kruger effect:

In the field of psychology, the Dunning–Kruger effect is a cognitive bias in which people mistakenly assess their cognitive ability as greater than it is. It is related to the cognitive bias of illusory superiority and comes from the inability of people to recognize their lack of ability. Without the self-awareness of metacognition, people cannot objectively evaluate their competence or incompetence.

The modern view on Saint Augustine is that he was purposely being manipulative:

Cousin marriage was once the norm throughout the world, but it became taboo in Europe after a long campaign by the Roman Catholic Church. Theologians like St. Augustine and St. Thomas argued that the practice promoted family loyalties at the expense of universal love and social harmony. Eliminating it was seen as a way to reduce clan warfare and promote loyalty to larger social institutions -- like the church.

I do not believe that it was a case of mere manipulation.

I rather believe that he was excessively convinced of his own abilities, and therefore, just stupid. There is obviously a reason why cousin marriage was "the norm throughout the world", before Saint Augustine, i.e. Saint "Mental Simplex", wrote that he "knew" why it had to be abolished in Europe.

Of course, Saint "Mental Simplex" did not know what he did not know, while intelligence is best defined as:

Knowing when you do not know.

"In the long run, we are all dead", John Maynard Keynes quipped in 1928. He said that about 1975.

So, when people can no longer identify with extended family, on the long run, what will they identify with?

Well, contrary to what Saint "Mental Simplex" believed, it is not the Church who became the beneficiary of his policy of destroying family ties. It is the State, along with an irrational and unsustainable identification with "race", who became the winners.

So, our beloved Saint "Mental Simplex" single-handedly triggered a phenomenon that would, later on, snowball into extreme racism and murderous nationalism.

The most spectacular "Oh my God, this cannot be true!" followed in 1937 when in their Papal Bull, "With Burning Concern", the Papacy decried the unstoppable emergence of a pernicious mental illness in Europe:

Whoever exalts race, or the people, or the State, or a particular form of State, or the depositories of power, or any other fundamental value of the human community – however necessary and honorable be their function in worldly things – whoever raises these notions above their standard value and divinises them to an idolatrous level, distorts and perverts an order of the world planned and created by God; he is far from the true faith in God and from the concept of life which that faith upholds.

Unfortunately, it was already a bit late in the game for the Papacy to frantically start bug fixing. As far as I am concerned, you've made your bed, now lie in it ... ;-)