I totally get the apprehension with respect to starting anew because, yeah, a lot of hard work goes down the drain. But at the same time, I personally always hit a point in minecraft where I feel I've basically "won" - I have stacks of iron blocks, a bunch of diamond, an endless food supply, a nice house, etc. At that point, what's left, really? I go explore some, maybe do some strip mining or build a new house, but really, there's only so much you can do with a world. As a consequence of this, I've barely been on smogcraft in months, and I suspect I'm not the only one.
I definitely agree that the most fun part of a server is its beginnings, and I think the update release cycle that mojang seems to be going for henceforth (i.e. huge releases punctuating enormous gaps of time) works really well for this sort of thing: several months of time, new features, start anew.