V A I N G L O R Y
There's nothing stopping a faction from going minor of their own free will. If a faction isn't willing to do that and can't maintain activity in a way that benefits the forum (as in actively posting threads which are directly attributed to the group as a presence on the forum in a way that interacts with itself and/or other factions/groups on the forum) then they get recalled. Making an arbitrary automatic expiration trades the status quo for factions manned by people who have a specific story in mind for a faction from start-to-finish, which is a fantastic method of running a faction but it isn't the only way to run a faction nor should it be.
A lot of people (as in most of the factions on the forum) have both historically and currently operated factions as a vehicle for people and groups to interactively write a story, not in the reverse manner. A few groups come to mind that have gone against the grain and done things differently, sure, but not everyone wants to engage in factions with the idea that their faction has to have a beginning-middle-end planned out and executed within a finite (known) timeline - a lot of people also tie their characters to their factions, not necessarily as tools for the faction to use but because it helps come up with a sense of identity for their character and, again, it is useful to use the faction as a vehicle to carry them along in their own personal stories that they otherwise wouldn't be able to come up with on their own.
Factions that have a finite timeline, with an arbitrary expiry period, runs the risk of pushing people who might otherwise be interested in a faction towards somewhere else (either private threads or a newer faction) so that they don't run the risk of investing into something that isn't going to last as long as their interest in their character(s), as Srina said.
This fosters an environment where factions, after a certain point, will be full of just the people who are most invested in a faction's success - which might sound like a good thing, but that's going to be a very small fraction of the amount of people who actively write in them right now. You'd see factions that aren't extremely popular or innovative enough knee-capped because the people who aren't willing to base a character around a faction nearing the end of their lifespan will pull activity somewhere else and interest in a faction that doesn't attract new faces is quickly lost which'll lead to an increase in recalls rather than whatever cycle this suggestion might intend for it to bring about.
A lot of people (as in most of the factions on the forum) have both historically and currently operated factions as a vehicle for people and groups to interactively write a story, not in the reverse manner. A few groups come to mind that have gone against the grain and done things differently, sure, but not everyone wants to engage in factions with the idea that their faction has to have a beginning-middle-end planned out and executed within a finite (known) timeline - a lot of people also tie their characters to their factions, not necessarily as tools for the faction to use but because it helps come up with a sense of identity for their character and, again, it is useful to use the faction as a vehicle to carry them along in their own personal stories that they otherwise wouldn't be able to come up with on their own.
Factions that have a finite timeline, with an arbitrary expiry period, runs the risk of pushing people who might otherwise be interested in a faction towards somewhere else (either private threads or a newer faction) so that they don't run the risk of investing into something that isn't going to last as long as their interest in their character(s), as Srina said.
This fosters an environment where factions, after a certain point, will be full of just the people who are most invested in a faction's success - which might sound like a good thing, but that's going to be a very small fraction of the amount of people who actively write in them right now. You'd see factions that aren't extremely popular or innovative enough knee-capped because the people who aren't willing to base a character around a faction nearing the end of their lifespan will pull activity somewhere else and interest in a faction that doesn't attract new faces is quickly lost which'll lead to an increase in recalls rather than whatever cycle this suggestion might intend for it to bring about.