This is another way to look at it.
It is economy that drove large family systems. Large families (large number of males in particular) meant ability to occupy large swathes of lands, manage large number of cattle and larger prosperity. Now it is the same economy that is driving small family systems. Lands and cattle have saturated. So larger family means larger poverty and hence smaller it is, easier to sustain.
As populations grow beyond the ability of natural resources to sustain, smaller families are driven down to individual's sustainability. More and more people want to remain individuals not having family, as it is easy for them to sustain, enjoy the pleasures of life without financial burdens. In particular more the economic independence for men and women, more this tendency to individualize. This means both men and women have lesser reasons to adapt to each other and remain a family. Yes, there are societal and family pressures still operating on them, but they are much less effective than before. Essentially it is a natural reaction to over-population on earth's resources.
So divorce rates are bound to go up. Marriages will come down. But as people become individuals, birth-rate would drop down. Once birth-rates drop down, average age of society goes up, younger people who can contribute to economy more effectively comes down. While the pressure on natural resources come down, the effective utilization of the resources also would come down.
This is where planet earth and its civilizations are now. Different civilizations are at different stages of this journey.
At some point, people will realize the need for families again to make better utilization of natural resources. So the tendency to adapt to each other will return to be a need and the norm. In some cultures staying along with parents will become the new norm. But it won't stop there. From small families, people may return to larger joint families to more effectively exploit the natural resources and this will cycle back.
But then there could also be technological hacks playing spoilsport in this cycle and take us on totally unchartered paths.
