The only way for the Dominion to fail is for humans to stand up together. If the Empire takes control it will do so at the price of the loyalty and lives of some of their best fighters, and will continue to bow to the Thalmor's demands in exchange for peace. If the Stormcloaks succeed, the Empire will have fractured completely and everyone would only care about their own little part of the world. The Thalmor sows discord because they know they cannot stand up to a united human nation. While we squabble they're free to move behind the scenes and work on their plots.
I actually think that might be the opposite of the truth. Historically, the most powerful men have always risen from chaos and desperation. Pelenial didn't come from a unified society. The Thalmor can infiltrate and manipulate organizations, but can they infiltrate and manipulate a band of barbarian freedom fighters? There's a chance that the empire could string together a military victory over the elves if they unify, but that'd be a real struggle. If they can summon another killer robot, that's it, and they're going to win.