My opinion
"Skyrim needs the Empire"
Translation-&--#62; We the Empire need the Nord armies to help bail us out each time our own incompetent legions from Cyrodill loses a city to those pesky Thalmor
Hammerfell having suffered the same devastation as Cyrodill during the Great War and having lost it's southern territories fought on alone for 5 years by themselves and drove the Aldmeri Dominion from it's land. I believe an independent Skyrim could also fend off a home invasion from the Dominion if the time comes.
What do you think?
I think you're on the right track. Skyrim has several lines of defense.
*Mountains and cold terrain. The Nords know the land and are adapted to it - the islander Elves don't and aren't.
*In every successful invasion in the real world, wherein a nation has to cross water to invade, a safe port has been absolutely required. They won't find one - Nords have been renowned for being excellent sailors since the days of Atmora.
*The Falmer were at the same peak of magical and civilized society as the Altmer are today, and 502 nords put them down.
*The Thalmor either have to sail past Hammerfell (good luck), those one Elves who don't like anyone from TESO, or go through Cyrodiil to attack.
*The Thalmor put the vast majority of their forces into the last great war and they reproduce more slowly than humans. Top this with that it takes a vast many years to train a high wizard and they have
nothing.*And last, but not least, the Nords were called the Strong Arm of the Empire and the Empire only started to repel the Thalmor once the Nords showed up.