Beth soft are always trying to be fair to all gamers, all fans, no matter the system they use for their games. Also they always try to produce the highest quality and we all know how fond they are of graphics and how much they put into progress in this field. Basically it means that they cannot release another TES game for this generation consoles and they will be reluctant to release another Fallout too, since they squeezed out all they could from current level Gamebryo. They will most likely aim for new hights and they won't do it just for PC gamers since that would be unfair, thus they will wait, end of story.
Except for the minor detail that Gamebryo's potential has not at all been maxed out, nor has the hardware for the current systems. Gamebryo is game engine software, and software always has room for optimization. Object culling, decent LOD texture transitions, FPS hit from NPCs on screen, etc, etc. If there were no more room for software optimization, then there would be no more versions of Gamebryo. And rest assured that Emergent certainly hasn't been sitting around doing nothing for the past 1 to 3 years. The version of Gamebryo that Bethesda is likely using right now as part of their in-house development title is almost guaranteed to be a higher and more optimized version than what was used in Fallout 3. And, hardware and memory optimization, writing more efficient shaders, etc, etc, means that your hardware doesn't have to be the latest and greatest to do great things with it. Current-gen hardware, therefore, is hardly maxed out.