Oh, I can totally understand that. But I think that there must be a moment when the modder will say "ok, this mod is pretty much finished, there is nothing else I'd like to implement". Or "this mod is working good and does what it says it does. I can keep adding things to it, but it basically works as I originally envisioned it".
I wouldn't know, because I never did any modding, so maybe I'm totally wrong here.

The thing about this is, there are different kinds of mods. New weapons and armor are unlikely to see updates: most of the work is the model and texture, and there's not much that's likely to change once those are finished. Quest mods generally have a plot that's written in advance, so they can hit that "does what I want it to" state and be feature-complete, though they might require bug fixes or compatibility patches in the future. Landscape mods fit somewhere in the middle; again, yes, they can definitely be "essentially finished."
But once you get into mods that change the rules of gameplay, rather than just what you're playing with, all bets are off. I thought I would be done with nGCD years ago, but a new version with some
huge changes came out in November, and I've got another set of revisions planned fairly soon. New options open up as modding technology develops, and old mods frequently need to be updated for compatibility with new stuff, or can directly take advantage of the new tools themselves.
You know how the games which show up on a console near the end of its life cycle are often extremely superior in both graphics and framerate to those which came out at release? It's basically the same thing. People figure out new ways to use the available resources, and can get Just Plain Better results. Because of this, a gameplay-altering mod is usually only "done" when the modder leaves the community... and even then, chances are good someone else will pick it up soon enough, either obtaining permission to continue the original or simply making it obsolete with something new.
By the way, while I've got the console metaphor up: you know they're still releasing new games for the PS2, right? The Oblivion modding community will not be killed by Skyrim, just as Morrowind modding remains healthy to this day.