How would that help a religion?
If new players join, it is presumably because the religion's ad attracted them. If they choose to become priests and invest in temples, then the religion is being revitalized: and that is a good thing. That's what we want. If they join the religion and do nothing, it will continue to die anyways. Nobles being in a religion do not keep it going. Nobles giving money and converting peasants keep it going.
