I agree with the points that
Delsin Shaw
made, but I think there's other arguments against it, too.
1. You die long before the end date.
If you're a new member and a Faction is on its last months, will it be worth investing time to make your character grow in that faction? Maybe if they aim to keep going as a Minor and that interests you, but I can also see it being a reason to already write off that faction if you're interested in a faction that can participate in invasions.
Cross faction RP can easily suffer, too. Invasions take time. 2 weeks of writing, 1 week of ooc sign-up and 1 week for judgment. That's like a month you can't speed up in any way for just a single invasion, and it doesn't even take into account the time it takes to plot between factions either.
If you want a bigger conflict between factions, would you want to commit to that if a faction has a few months left? Nah that faction is already dead, so unless they want to use that story to die off, it won't be interesting.
Then there's the issue that a lot of factions also slow down, well within just their first year. If you commited to a story with one of those and they fall off, you lost a chunk of your timer and the story didn't go anywhere satisfying.
2. People come and go
While Factions last for years, most people don't. The GA, as an example since it's the oldest Major, might be 5 years old, but I've been part of it for a little over half that time. The people leading it when I joined are gone, and even they were not the folks who created the faction.
Currently I even have someone on my staff who joined the site only 6 months ago. What's old and "overdone" to oldies who joined in 2013 isn't the same for everybody. To new members, it's a new story, their first faction, and their first time interacting with the map. Their activity speaks volumes, in my opinion, and the history of the faction they join can make their stories rich.
Factions don't last on the same few people doing the same thing over and over. And artificially telling a lot of new folks their time is up because they joined later during a factions lifetime is not the way to generate RP.
Especially because newer folks benefit a lot from first joining an older faction, so they can gradually learn how things work before they make/run something themselves. Even older members sometimes don't know how the map game works with the various types of threads etc.