My girlfriend and I actually had a discussion about this last night, and this is what we concluded with:
Despite appearing weak by agreeing to stop worship of one of the divines, the Empire is simply on the verge of collapse as opposed to in the middle of falling apart.
Based on a few random NPCs (like the guy who the dark elves like in Windhelm) the "Great War" was horrible with both sides suffering many casualties. By signing the peace treaty, the casualties stop. The Thalmor are now (mostly peacefully) interacting with the residents of Skyrim. Over time, they may come to realize living in peaceful coexistence is much better than chancing utter annihilation. Or, at least the Empire will have time to prepare new troops (Since the Empire consists of multiple races and countries, I would imagine they would be able to accrue more troops quickly... but I don't really know)
While the Stormcloaks seem to have pretty righteous reasons to fight against the Empire and revolt (Revolutionary War style), it's extremely short sighted. If they are successful, Skyrim loses support of the Empire. As spread out as Skyrim is, plus accounting for casualties in the revolution, Thalmor could pretty much storm in and conquer the province and the Skyrim...ians? would be rather helpless. Even if they decide not to attack at all, the Empire brings a lot of trade up to Skyrim; without the Empire, who knows if the economy would be able to hold.
On the Empire side, they would lose the only benefit of the treaty (preparation time) by having to send more soldiers to fight with the Stormcloaks. Thus, the Thalmor would most likely go in for the kill.
In summary, Ulfric Stormcloak needs to svck it up and help the Empire rebuild so they can fight the real enemy and stop whining about what the Empire did to save everyone.
Talos ascended to God-hood once. I'm sure he could do it again.
And that's my two cents!