All too many FFBs lead their lives along the path of least resistance. They were raised frum and stay on the derech mostly by inertia. When uprooted from their friends and family, many decide the path of least resistance isn’t the derech of Torah. Look at what happened to East European Jews coming to America, or having their lives turned inside out by the first World War. R’ Aharon Kotler, R’ Yaakov Kamenetsky and R’ MM Schneersohn had one thing in common. Of all their brothers and sisters, they were the only ones to remain shomer Shabbos.
On the other hand, BTs find that the z’chus of teshuva gets amortized over the years, so eventually they feel very much like a stam Yid. They may still have a strong sense of bechira, but it’s very hard to transmit it to their children.
In the future it will be very easy to tell BTs from FFBs by their tattoos.