TES:VI? I doubt they will make it, it would make no sense. TES Online is coming and with it all provinces. Skyrim was made for it's final showing, why you think all this effort was put into it? How can you make something better than the Dragonborn anyway. Solstheim, Hammerfell, Cyrodiil, and High Rock are already coming as expansions. No TES VI buddy. Too much to ask for imo.
Assuming TES:O is popular enough. I would like to see a TES: VI but I can see your points

It wouldn't matter if it's popular enough. Saying there won't be an Elder Scrolls VI because it has all the provinces moots the entire Dragonborn DLC. We've already been to Solstheim in a DLC. Bethesda making a DLC for Solstheim again makes no sense. (See what I did there?) Also, where are these showings that we're going to Hammerfell, Cyrodil, and High Rock as expansions? We've already been to Hammerfell, Cyrodil, and High Rock. Making DLC for those places we've already been to makes no sense! Skyrim is not Bethesda's "final showing", nor is it far from being the last Elder Scroll game. Skyrim's popularity and success has definitley secured that. The Elder Scrolls series is Bethesda's flagship series. That'd be like Bioware not making another Dragon Age or Mass Effect (which they are). Also, as far as "all this effort" goes, that's pretty subjective, considering you could look at Oblivion and say the same thing. Or even Morrowind. Not to be rude, but saying Skyrim is the last Elder Scrolls game is a pretty naive thing to say.
You know thats only for PC right? Its and MMO. So TES: VI isn't out of the picture there are still us xbox and PS3 fans that need a game to play and as popular as Skyrim is I know everyone is wating for TES: VI
And that's another very good point. Bethesda's flagship platform now is obviously the Xbox 360. Plus, in retrospect, Lord of the Rings Online pretty much includes a majority of the places in the LoTR world, if I'm not mistaken, yet they still make other LoTR games. Just because one game that is for the smaller of the 3 platforms main Elder Scrolls games are made for includes all the provinces in no way means that they're just gonna stop making Elder Scroll games after TESO