As a long time Finnish iSteve reader, maybe I should comment as I have some personal experience in this matter.
I beleive the rapid decline in Finnish birth rates can be mostly explained by two development.
1. Economic development favoring big cities.
2. Immigration from the Middle East and Africa.
Nokia’s decline and the closing of paper mills took away a lot of manufacturing and R & D jobs from cities with 50 000-200 000 inhabitants that were locally important.
Now everybody feels the need to move to Helsinki, but because of immigration, living in neighbourhoods with “good schools” is very expensive.
I think this situation has resulted in some kind of culture shock for Finns, because this used to be a very egalitarian country, meaning upper middle class kids could go to school with poor kids. Finns have a hard time adopting the snobbishness that would be required to protect your family in a multicultural environment. This culture shock can explain the rapidity of the development.
So in our Brave New World of Winner Cities and Loser Towns, Finns needs to crowd into metropolitan Helsinki. But Finns haven’t yet developed all the complex adaptations that, say, Americans have developed to deal with metropolitan diversity: private schools, charter schools, magnet schools, private security, gated communities, gentrification pushes, sports travel teams, etc. Life in Finland has tended to be fairly simple: you live somewhere, pay a lot of taxes, get a lot of services, and participate in the local community. That worked fairly well with a whole bunch of Finns around.
Finns aren’t stupid, so they’ll likely eventually work out work-arounds to diversity. But so far, Finns are freezing in the face of diversity, and their fertility is falling.