This is also true of some ultra orthodox sects of judaism that are thriving.
Also, the edict is only since 1937, they were accepting converts earlier than that. So fundamentally they aren't a success story - they haven't been doing this very long.
Other Jews can marry in, but even this is limited. The majority of self-identifying Jews in the US would not have the qualifications to marry in. Any how, they are an insular community.