I think the Witcher 2 redkit thing is also a way of modding stuff. The biggest influence of how moddable something is, is depending on how many people are playing the game, loving it and want to mod it. Now I did try Redkit a little bit. Managed to make some hills, a house, a bridge and managed to walk, then I quit.
It is not that user friendly for people that have no modding knowledge. What is also crucial is time of modkit release. The redkit release was done waaaay later than when the game came out. Skyrim came after Witcher 2 and a lot of people's attention had gone over there. I think a modding community keeps a game alive and makes people want to keep playing and attract more people to buy it. I mean minecraft is still selling. The modding community there have transformed minecraft into more than the original 'vanilla' version offered. This means more content, more options, etc. -> More people wanting to buy the game.
But yeah we don't know how moddable W3 will be. I do think that W3 will not be able to be modded as easily as skyrim. However I judge that on the fact that Skyrim is more of a sandbox game like minecraft.
Or maybe skyrim was so stale in comparison to the witcher that people desperately tried to make it something good. They did not need that for the witcher 2. =P