I know of dozens of churches, personally, and hundreds of others, statistically, that survive only based on the fertility of the couples raised in the church. When they hit the point that the people in the church pass child bearing years they begin a cycle of inevitable decline, usually coupled with a generally aging of the community. It may take decades but they eventually die. Maybe sad, maybe natural, but it happens.
However, the Atlantic article linked above tells us about an entire faith system that is feeling the strain. The Zoroastrian faith, or so some of its leaders fear, is on the verge of assimilation and death. Part of this, as is often the case, is in part to tradition. "Zoroastrianism is a patriarchal tradition, so the children of Zoroastrian women who marry outside the faith are not accepted, and even shunned, in many communities. Meanwhile, children of Zoroastrian men who intermarry are likelier to be accepted." So, if your daughter doesn't marry in the faith, your grandchildren aren't considered Zoroastrians.
The faithful practice private devotion, lacking the public worship centers of other world religions, which leads from a purely practical view to a lack of a touch point in the community. Some curious seeker could wander into a church, synagogue, or mosque, but that is not going to happen for the Zoroastrians. Outside of India and Central Asia the numbers of Zoroastrians are so small they are usually lumped into surveys as "religious other" lacking a critical mass needed to sustain themselves.
So pressure falls to the children of marrying age to stay within the faith in an increasingly secular world.
While unlikely to be an issue in the immediate future for most of the faiths that are a little higher up the percentiles on the surveys, it is something to think about. It certainly happens on a smaller scale to individual congregations. What changes can be made to insure existence but keep identity? What lines cannot be crossed, even if it means the end of the faith?