But the economic problems followed the fundamental changes in people's attitudes towards basic matters such as marriage and personal morality. Not the other way around.

No, it's the other way around. Economic changes result in changed morality.

Rome's problem was over-expansion. Their strategy was conquest, and it worked for awhile. Their farmer-soldiers could farm most of the time, and go to war occasionally. But as the empire got bigger, that became impossible. Soldiers were stationed in places like England and Germany - too far to come home for planting season. They were caught in the classic guns or butter bind. Their economy relied on the inflow of gold and goods from conquered terriitories, so they could not easily give them up. But they needed food, too, and the people who used to farm were now full-time soldiers.

Another example of economics dictating morality was London in Dickens' time. Economically speaking, it was nearly impossible to support a household with more than three people in it. Even the middle class could not afford more than one child at a time. So if a second child came along, they either killed it, or kicked the older child out of the house.
You see both at work in Oliver Twist. Orphanages were church-sanctioned baby-killing organizations. More than ninety percent of children in such orphanages died before they reached age 16.

If the parents kept a new child, the older child would be forced out on the streets to fend for itself. Children as young as three were turned out of their homes. Hence the gangs of boys roaming the streets, which we also see in Oliver Twist. (The girls were usually sent to work as servant girls or in brothels. They were also more likely to be killed at birth than boys.) Many of the street boys eventually ended up in the British navy, victims of press gangs.

Definitely not what we would consider a family friendly society, yet economically, they did very well.

But extended families are merely -- as the term "extended" indicates -- formed from the building blocks of nuclear families.

Not true. If it were, the nuclear family would be a universal, found in all human cultures. It is not. It hasn't even been the norm for most of our own history. "Paradise Lost" syndrome doesn't just refer to the Noble Savage myth. It also applies to those who romanticize our own past. The nuclear family was the rule in the '50s, which was also an economically prosperous time for us. Assuming the former caused the latter is not supported by the facts.