Well considering that Skyrim is in the third century of the fourth era (the third era being around 450 years long), and the love letter is only in the fifth era, honestly, the technology mentioned and stuff seems odd to be so close, so much so that I'm tempted to call the letter non-canon, or at least the date on it incorrect.
EDIT: Ok, I just noticed that the letter is around 900 years into the fifth era. This seems very feasible now. I'm still very curious to see what will happen because of Alduin's not ending the Kalpa.
Aside from that, I don't think we know how long exactly the fourth era lasted, for all we know, it could be longer than the first,
Still, I hope we never get to see the future descibed in the Loveletter come to pass, not because I necessarily object to the thought that it describes a canonical future to the Elder Scrolls series, but rather because I want the series to preserve its Medieval-ish fantasy feel, if Bethesda wants to do science-fiction, they should keep it to Fallout, or start a new series if they want to try concepts that wouldn't fit the setting of Fallout, for the Elder Scrolls, I'd prefer to keep any advanced technology we see to the Dwemer or Sotha Sil.
However, what exactly will happen should the Dovahkiin stop Alduin from eating the world (Which I think is pretty much guarenteed unless Skyrim has multiple endings.) is something we'll just have to wait and see, maybe a dragonbreak will happen, maybe we'll find out the reason why Alduin eats the world, or maybe it will just turn out that things go on just fine, but seeing as Bethesda seems to like to have the players' success in one game lead to potentially unexpected concequences in the next, perhaps the same will be true in Skyrim, but as to what form that could take, perhaps we'll have more information to speculate on once we've had a chance to see where the story goes.
The Empire will both completely lose power in Skyrim and witness the foundation of the glorious Skyrim Kingdom, AND, fight back and crush the rebel scum and have its influence vastly expanded in Skyrim. Gotta love dem dragonbreaks.
In short, don't think too highly on the implications of a dragonbreak, or you'll get a serious headache.