The failure to have replacement level populations is only a problem in which there is a large social safety net that must be paid for by a work force. Europe could exist for centuries with declining birth rates if it didn’t have a large welfare state hanging around its neck.
Small population growth is the number one sign that you are a modern, advancing state.
Why did people have large families in the past? For two reasons. One children died a lot. Having ten kids ensured that you would have five children reach adulthood. The other is that children were cheap labor. They paid for themselves by working on the farm (or earning money in a coal mine or something).
There were practical reasons for large families that simply do not exist now.
Okay, I agree with that as far as it goes. I think your rather naturalistic explanation ignores the fact that we do have a welfare state that must be paid for. It also ignores the fact that a modern economy is based on consumption (70% of the US economy is driven by consumer spending) and that it is precisely young people with families who are the big consumers. Old people are not. It's IMHO a big part of our current predicament - i.e. we're trying to address the recession with methods that worked when the baby boom was young and in their prime, and not when they're old and they're spending like old people.
The only fix for these problems will be to import young people. And that's what we've been doing. For us it will mean Hispanics. For Europe it means Muslims. I prefer the social ramifications that we have. God pity the sterile Europeans.
But if we "social conservatives" are animated by common biblical principles, then all we need to impel us to have multiple children is God's mandate to "be fruitful, multiply and fill the Earth".