Conjecture. Even if that's somewhat true, there are plenty of extraneous factors that affect numbers and the culture that creates the numbers. The economy, shifts in religious attitudes, shifts in cultural attitudes, etc. Divorce was unfathomable to lots of people a few decades ago. Now it's barely stigmatized. You have no evidence to suggest that peoples' relationships in earlier times were more stable and more successful just because they got married and didn't get divorced.
see the review