Starglim gives the most likely scenario for any sect of worth.
In my Homebrew's current regional setting, there is a small sect, that of Miros, the Goddess of Light and the Sun ("Our Lady of the Light") which is in jeopardy.  There is only one Cleric of 9th level or higher (the old "high priest" level) who can cast Raise Dead or other 5th level spells necessary for the proper functioning of a regional sect (ie, Atonement, Commune, Hallow, etc).  This Cleric is in his late 80's and has no obvious successor.
For my next campaign, I have it written in that he will die of old age about 4 years into the campaign.  If there are no PCs of that sect of 9th level or higher (or NPCs aided by the PCs to the point of reaching that level), then the sect will fall into 'cult' status.
At that point, the sect will begin a death spiral, losing worshipers and new recruits to the clergy, until it dies out in this region.