Could modders make a better TES game without the use of the devs toolset(Creation Kit) and only their own creativity and assets to come up with a game within a reasonably timeframe that is better than the one BGS made?
Seriously? I sure as hell do not think so. Nehrim looks great and all but they used the CS to make it.
Modders are great but they are not dev teams. They can make new worlds and quests given the tools Bethesda gives them. But seriously look at the mods out for Skyrim now without the Ck and compare them to the mods done to Oblivion with the CS. There is a big quality gap and they are incredibly limited by what they can do without the CK. The modders can do amazing things even without the CK but they need the thing to make any mods that really blow the game away.
You do realize that the people who put the game together do so with the construction set, and that the people who code the construction set are not the same people who put the game together?