Alan Bittles, in his summary of consanguinity research, notes that consanguinity prevalence is currently rising:
…as greater numbers of children survive to marriageable age, the traditional social preference for consanguineous unions can be more readily accommodated.
Looking into the future, he predicts a reversal (my boldface):
Irrespective of prevailing legislation, a future decline in the prevalence of consanguineous unions can be predicted, accompanying the expected reduction in family sizes. It seems probable that this decline will not be uniform in effect across populations but will be mainly observed in urbanized populations and among couples who share higher educational standards and later ages at marriage. The specific type of consanguineous union contracted may also prove to be an important determining factor.
Bittles also notes that consanguineous unions tend to increased fertility:
A partial explanation for these findings is the generally lower parental age at marriage and the age at the first birth of couples who are close relatives (Bittleset al. 1991, 1993).
When taken together, these effects would constitute a positive feedback loop that could push populations into a spiral of mutually reinforcing decreasing levels of consanguinity and fertility once critical thresholds are crossed.
The thought that such an outbreeding ratchet could actually be the hidden cause behind low western/urban fertility levels does not inspire confidence in the reversibility of fertility trends and future viability of these societies.