The modders saved Morrowind, and the modders saved/will save Oblivion. As far as I'm concerned, Bethesda simply creates a decent world and then passes on the tools to the modders.

I have no doubt that the modders will make more good stuff for Oblivion, but it won't make me pick up the game again - after "completing"(guilds+main quest) it twice, I've had my share for a very long time.