What is wrong with invei hagefen veinvei hagefen? There are some BTs who could never marry FFBs because they retain much of their old culture – and these are often davka the ones who complain they can’t find someone when there are tens of mates for them who are also BT and haven’t given up their yoga, or their Grateful Deceased, or their nonconformist dress etc. And there are some old-fashioned families who want to marry into families they knew in the alte heim – so be it (until it gets to the point that there are genetic problems LA from a small gene pool, but Dor Yeshorim helps with that).
Some BTs kvetch about everything in their new world too, and think they are the ones who should make changes before they even know alef beis. FFBs are only right to see BTs as unstable when week after week, these types (dis)grace their Shabbos table before they drop out anyway. Believe me, it is no fun to have your Shabbos disturbed by wannabes who call you racist, fascist, misogynist etc because they don’t like the answers to their pointed questions.