If you complete the civil war with the Stormcloaks, before his execution he suggests that such a revolution is what the Thalmor have been waiting for. So that both endings to the civil war are possible, the Thalmor will most likely take Skyrim from either victor and make the conclusion to the civil war obsolete. The prediction of many is that the next ES game will focus on a war with the Thalmor, who have gained power over the course of the last couple games.
How? The Thalmor would have a hard time reaching Skyrim. They'd either have to march and army through Cyrodill or Hammerfall. That or sail around to reach skyrim. Likely harrased by Hammerfell and High Rock the whole way. If skyrm remains with the Empire then they can send reinforcements from both Cyrodill and High Rock. If Skyrim is independent, Ulfric may send his forces to fight the Thalmor but I doubt he'd just let the Empire march its armies into the kingdom.
No what that Thalmor want is continued Civli War as that weakens there enemies. They would prefer an independent Skyrim because two enemies with two agendas are easier to deal with then one big enemy. But a swift victory for the Stormcloaks won't weaken there enemies enough.
Medes is afraid of the Thalmors. Tullius goes round implying they'll be having a war soon but nothing is ever done. If you let the Imperials win they'll all keep pretending they're only biding their time to recover for another war. 50000000000000000 years later they're still saying the same thing. Ulfric on the other hand is actually taking immediate action. That's the kind of man who should lead.
So your expecting years of timeline to be worked out an major world altering questlines beyond the main quest and the civil war. If you wait two hundred years in Morrowwind. Oblivion and Skyrim don't happen. So your entire logic is [censored]. Ulfric doesn't do anything either because there is a limit to how far things can go in a video game.
But after reading stuff about how much Tullius is in bed with the Thalmor it sickens me.
Where did you read this made up digest? Ulfric is actually listed in Thalmor records as an unwitting ally. The Tullius is required to play "nice" because there is a treaty and he has to enforce it until the Empire is ready for war again.