I know a lot of people stayed on with Morrowind when Oblivion released, and I'm sure a lot of modders will remain with Oblivion when Skyrim launches, but I think more mods get seen and played with the long waits we have for this series, and that the huge overhauls the developers provide us with in the new games are worth the wait. A new release every 2 years... doesn't really sound that appealing to me. 4 or 5 is about right I think.
The problem with your logic here is that WAY more people play these games on consoles than they do on PC. Consoles are where most of the money's at, and mods aren't available on consoles... It's not wise to make your main money source angry and impatient...
If I had my way Bethesda would ship the Fallout IP to a different developer and focus solely on The Elder Scrolls. That's not gonna happen though... Maybe Bethesda could continue growing, split their development group in half, and work on Fallout and The Elder Scrolls simultaneously...