I remember when I first started playing that the revision lengths were approximately three months, which I liked. I think the renewal process is important to holding interest.
That having been said, I want to raise the possibility that declining server population density may (at least partly) be a result of Minecraft aging as a game. As it is, Minecraft has been incredibly durable as a game, but people eventually move on to other games. There may not be anything that can be done to fight that