While I agree consider this -- a whole new engine means a whole new set of tools for Bethesda's staff to learn, in addition to engine licensing costs. There's a huge cost overhead for Bethesda to move to a new engine, which is why we see the 'Creation Engine' for Skyrim: it's easier to tack on band-aid 'upgrades' than use a new engine from scratch. Ultimately, though, the Gamebryo base is showing its age and at some time Bethesda will need to switch and foot the cost for future profitability.
However, don't forget -- as modders we've been able to do so much modding and port work from previous games precisely because the engine changes little with each new release. If Bethesda switches to an entirely new engine, depending on its license requirements, modding may be much more limited with the new engine or not be possible at all. Two-edged sword eh?
I know. But this is a case where we are going to need to decide whether to move with the times, or if the times will move us. All good things must come to an end, and eventually we are all going to need to learn a new engine to keep this series going. It's not pretty, but its the only way forward.
One place that Bethesda could start looking is CryEngine: open-world, pushing the graphics envelope, and easily moddable. If Bethesda can take a few leaves out of CryTek's book and create a similar engine with the flexibility they need, that might be enough to insure TES VI is just as impressive visually as TES IV was back in 2006. As for the cost... I think Skyrim will take care of that on its own.
And now, back to our original topic.