My guess is that TES VII will be the last one. In TES V and VI, the series will probably change so much that much of the hardcoe fanbase will lose interest. They will be stuck with casual gamers who will only care about graphics and gameplay. This position can be quite difficult for any company, because you may pump a lot of money into graphics, but if you don't get enough press coverage (or if another game gets too much coverage), then your game won't sell as well. hardcoe fans give free publicity to a game, which can help sales quite a bit. Based on the current trend, TES games will only get more simple as time goes on. We will probably see: fewer quests in exchange for longer, more elaborate quests, fewer dungeons (I'm guessing ~20-25 for TES VII) in exchange for longer, dungeons with hand-placed loot/enemies, fewer skills, etc. In interviews, we will probably hear things such as "Yeah, those older TES games were too complicated. They had so many skills to choose from and so many copy-and-paste dungeons and quests that they simply did not hold the interest of the mainstream. This streamlined design makes TES VII a much better game!"*
Of course, this may be an example of the slippery slope fallacy, but I can see it happening.
*Anyone else remember how Todd's defense for all of our complaints seemed to be "But it's a much better game, now!" when TES IV first came out?