And it wouldn't take forever- many weapons and items from the old games have already been ported to Fallout 3, so as long as I (and/or anyone else who would work on the mod with me) could get permission from the authors of those mods, we would really only be doing some mostly minor modling. The hardest part would be new (or rather old) NPCs and creatures, probably, and many of them have also been ported to Fallout 3.
You're massively underestimating the time required to make + clutter cells and worldspace, set up dialogue and quests, set up packages, testing the world and particularly navmeshing. The content meshes aren't really the limiting factor. Tamriel Rebuilt, for example, has pretty much most of the meshes it needs- it's got plenty of barebones villages done, but not very many actual quests, not much fleshing out done.
This is not least in part because meshing is fun but ancillary work is not- I'm speaking from experience, I was on a Morrowind TC (name best left unsaid) for a while with all the resources we could ever need meshwise, and after 3-4 months I'd gotten the AI packages done for all the villages, and just gave up. I pity the poor bugger they've got doing the AI for Vivec.