Fair enough, my only point earlier was it was in their capability, but I'd be happy to discuss whether they "would" do it.
Well, for starters, I would do a whole lot of things very differently from Bethesda, so I will try to keep this limited to what I think they would do, or we might have a much bigger discussion. As far as what I think they would do here, it seems to me that they have a pretty strong history of recreating the game from scratch every installment, and if I were them, I would keep doing that, since its a formula that's kept the games fresh (despite my many complaints about streamlining and cutting of gameplay). So, "start fresh" philosophy would preclude TES VI from being centered around war with the Thalmor. If they wanted to wrap up the conflict with the Thalmor in game, then they may want to do it with a Skyrim expansion, so they could start fresh again with TES VI.
Given the length of time between TES v and VI, an expansion to V might sell pretty well if it were big enough to be considered a whole new game. So, yeah, they probably would do it if they thought a major expansion to TES V would sell well enough to recoup costs and make a sufficient profit, assuming they have staff to work on it, who perhaps may be between other bigger projects, etc. Lots of unknowns and assumptions here, and none of us really really have access to enough information to do anything but guess. But assuming the stars aligned and Bethesda had staff available and thought it would make money, then sure, I think they would do it.
I think they could wrap up the conflict with the Thalmor, not by invasion (since that does not seem plausible currently) but through a collapse or coup that happens from inside the Summerset Isles, with the Dragonborn acting as a catalyst. Who knows, maybe the Thalmor have figured out a way to unmake the world but the dragonborn prevents it at the last moment and the Thalmor are swallowed up or unmade themselves in the cataclysm that occurs as a result of their failed attempt that was spoiled by the Dragonborn. That's just one possible scenario, and probably not the best one, but there are a myriad of ways Bethesda could write an end to the Thalmor story without needing an "invasion."
Now, to answer your question of whether they would "burn" Summerset Isles with an expansion, I think that depends on how much money they think could be made with a province sized expansion to TES V. If they project enough money to be made, then yeah, I think they would "burn" it. They have many provinces that are as yet unseen in any of their modern games, plus other continents that have not been used at all in prior games, so its not like they are going to run out of real estate any time soon.
Also, just because they take us to a province in one game (or expansion thereof) doesn't necessarily mean they won't take us back there in a future game set in a different time period. They have already done that with the Isle of Solstheim, which was the subject of a Morrowind expansion (Bloodmoon)and then repeated in a Skyrim expansion (Dragonborn). I see no reason why they could not do something similar with a province.
They also would not need to "burn" the whole province of the Summerset Isles in order to take us there to wrap up the Thalmor story. There are many isles and they could confine it to one of them. We have actually never seen mainland Morrowind in any game. TES III was set on the adjacent and recently reopened Isle of Vardenfell, and the only time we got to mainland Morrowind was the Tribunal expansion where your were confined to the walls of a single city (Mournhold). We never got to see Morrowind mainland. They could do something similar with Sommurset Isles for a Skyrim expansion, by confining it to a single island instead of showing us the whole province. That would preserve the rest of the province as pristine virgin territory for a future game.