Why?
I don't think that Bethesda is stupid enough to release a game without mod support. The reason that Fallout 3 sold like it did was that people fixed the broken aspects. Skyrim nearly became a money printing enterprise for Bethesda nearly 4 years after release. New Vegas came to the party way too late, and only had staying power due to community mods. It doesn't make any sense for Bethesda to release a game without mod support. Even their cynical executives would see that as cutting the life of their games, and subsequently their revenue stream.
What I see is Bethesda making modding easier, rewriting their TOS, and beginning their own mod market. If they introduced a certification process they could sell mods, pull a 25% fee for each sale, and put all of the liability on the modders. Assuming they start on day 1, with all mods forced to submit to their TOS, there's no problem using existing assets. Bethesda gets to print money with very little work, they claim it as a win for modders being paid, and they realistically can make money off the game for years. Seriously, who plays Skyrim vanilla?
Yes, Gamebryo allowed modding because it was archaic. Yes, it would take effort for Bethesda to make Fallout 4 mod friendly. At the same time, a few extra hours of work during development would allow huge profits for years to come. I can't see an executive saying no to that proposition.