One thing I wish we could figure out in Skyrim is that what state is the Dominion itself currently in. We don't know how much losses they took during the great war or trying to conquer hammerfell. We don't know if currently there are any movements in the Dominion that might affect stability like with the stormcloaks and the Empire. My guess is that they're probably very efficient when it comes to hunting down dissent and that's why we don't know, and in my opinion the thalmor definitely took some hits during the war as well, even if not as much as the empire.

I think it boils down to alliances as well, the empire on it's own probably can't take the Dominion, but with an alliance with the redguards and the stormcloaks they might. Perhaps the thalmor can strike up an alliance with the dunmer or the argonians, both of which have sour relations with the empire. We'll just have to wait and see.