Nah, not really. They haven't been claiming that Thief 4 will be extremely moddable or anything like that. Nobody is going to produce anything on the Dark Engine that competes (technically) with a modern game engine and a full, devoted team. I'm not saying people can't make better missions with an inferior toolset, I'm saying that even if they did it won't appeal to the mass market. It won't steal any thunder from Thief 4.