The other big thing to consider is that -- according to various interview comments from articles over the years -- Todd Howard and his team are equally interested in pursuing non-
Elder Scrolls-related projects in between production of the major sequels for that series. It's the main reason why they worked to secure the
Fallout IP from Interplay in the first place, and it wouldn't surprise me one bit to see Bethesda Game Studios hand off the series to a sub-developer for a side-quel type game of some sort, after TES V is completed.
BGS themselves will either be working on another
Fallout, or else something brand-new entirely. They're not wanting to get burned out by doing a non-stop constant stream of
Elder Scrolls games.