It's interesting when people can't look outside the box that is placed upon their head while opposing to more potential options of what one may do with a product. Redkit for The Witcher 3 also would open up a current tool in the game industry for environmental design with it's terrain/landscape editor. Not to mention all the animation capabilities... A decent amount of modders of games in the past have become game developers themselves. Some talented people out there would prefer incorporate their artistry within a good existing game rather than attempt to start from the ground up. There are game engines like Cryengine, Unity, or UDK but they are all suited for modern FPS gameplay.