In all honesty - the Empire as it stands will eventually fall. Nothing lasts forever.
I, however - do not agree that the Thalmor will be its harbinger of destruction.
What do the Thalmor want? The eradication of all human races. Redguards, Imperials, Bretons, and Nords.
Hammerfell has seceded from the Empire, if I remember correctly. Skyrim and High Rock are set to do so, as well. However, even if the Stormcloaks win the war in Skyrim - I wouldn't count out the Empire.
Leaders of governments are often quite intelligent. If they aren't, they have advisors that are. There is simply no way in hell that Hammerfell, High Rock, and Skyrim would sit idly by if the Thalmor marched on Cyrodiil. If Cyrodiil falls - their nations are next. They will without a doubt know this.
If I remember correctly - the Dunmer dislike the Thalmor, so they might not even join the struggle. Even if they did - they are an exhausted, and battered people. I don't think they have much of a military left if they have one at all.
The Khajiits are contempt in their deserts and jungles. Screw them.
The Argonians? They'll likely aid the Imperial forces. For the same reasons they conquered Morrowind. The Empire abolished Slavery - after all.
I don't remember hearing anything about the Orcs, however I imagine they'd aid whomever paid the most.
I foresee another treaty in the works. In exchange for helping Cyrodiil defeat the Thalmor - each nation will have the option of seceding from the Empire if it so wishes. I think this is how the Empire will crumble.
Just my random thoughts on the discussion.
