Recall also the Gygaxian expectation of "10 levels = 5 year's play".
RC
So a campaign was suppose to be balance over the course of 5 years?

I don't think most groups stayed together that long. Typical Gamer starting when 14. Fours yeas then collage, jobs and adult hood. Collage max 4 years before jobs, how many people live in the same city for more then 4 or 5 years?
Of course Gygax had know information except his own games to base any thing off. There was I time period where I moved or changed groups because of other people moving once every year and a half. How as Gary to know this was common to a large segment of the gaming community that is military?
The change in length of campaign is driven to some extend by our culture and economics.
Now some of you can do 2+ year campaigns. For you a new game or a rewritten 1DD type game would work. For lots of us it won't.