Not trying to be contentious, but Witcher 3 and DA:I may both have better rendering but their respective worlds are more limited in the amount of layered details that are present within the game world. I'd like to see anybody pick up a random pot in Witcher 3 or DA:I and move it around. Personally, the rendering on both of those titles did not fill me with the same level of awe that it has some people. I will not discount Witcher 3 as a beautiful game but I also will not say that it looks "better" than FO4. DA: I was pretty fugly to me. I hated the super sparkly textures that were used for armor and whatnot. I saw much beauty in the environmental stuff, but it just didn't seem all that great with character textures and animations.
All that said, I don't think that Bethesda will abandon the Creation engine anytime soon. I imagine that they may gut and rebuild some of the limiting portions of it, but to get rid of it would make future Bethesda ARPG titles seem less like Bethesda games.
Personally, I would rather they stopped making idiotic design decisions. They made the same keybinding decisions with Skyrim and received the same backlash. Fix that and the UI issues and Fallout 4 would score near perfect in my book. Right now it is an 7.75/10 for me. I'm having a blast with it, but the above mentioned issues are detracting from the overall experience. That has little to do with the engine itself and more with design decisions though, so probably do not have a place in this thread.