If there are seriously people refusing to even give The Dark Mod a try simply because it's based around fan missions instead of an "official" story arc, I can only strongly assert that they're missing out.
The vast majority of the 1000+ Thief/Thief 2 missions released to date have little to nothing to do with the original story, and I don't mean stinkers -- I'm talking about the highest rated big missions and campaigns. That is in of course in part because fans who can produce such work invariably have creativity and will of their own and want to distinguish their work.
TDM now has something like 70+ maps. A good chunk of them are campaigns and series. It's got more than one story; it's got many! Just like T1/T2 FMs. But I guess if you're not into playing FMs then that's a problem of a different type (not knowing or accepting that a great many of them are better than original missions, and sometimes land the authors in game Dev jobs...)