The Reform Jews of suburbia today often have grandparents from Eastern Europe a century ago that were orthodox.
I'd be curious to see the stats... Most modern US reform Jews are the descendants of European reformed Jews, as I understand it. In any case, a girl born in a ultra orthodox community doesn't seem to have better prospects than a girl born in Iran, regarding jeans and modern music... Or re. changing her religion for christianity, islam or buddhism...
You are wrong. The German Jewish migration in the mid-nineteenth century was not that large. It was part of the immigration of Germans after the Revolution of 1848. A percentage married into Christian families before the turn of the 20th century. Those that remained Jewish were Reformed, since the Reformed denomination started in Germany.
The big wave of Jewish immigration was the Russian Jews of the late nineteenth century, and early 20th century, before the gates were closed in 1924. They were mostly orthodox, not Chassidim. Their children often became Conservative, and their children might be Reformed today living in suburbia. So, by sheer numbers alone, most Reformed Jews today are descended from the Russian Jewish immigration between 1880 and 1923 of originally religious (Orthodox by today's definition).
I believe a majority of the Orthodox today are descended from survivors of the Holocaust that came here post WWII. Same with the Chassidim. They live in a community that allows people to leave, if they choose, but if one stays in the community, certain ways (jeans, etc.) would be like breaking wind in church - it is not done, or one might be asked to leave, or at least decide if one wants to remain within the community? So, since the Orthodox community maintains standards for their adherents, it is silly to talk about whether one can wear jeans, and still be Orthodox. Sort of like saying why can't one be a librarian but be illiterate. The incongruity of behavior that is alien to the Orthodox community just means that it is a closed club for those that subscribe to the standards; however, there is no legal way to stop someone from leaving the community. If you are not aware, the laws in the U.S. do not allow such actions between adults.