Yes the Empire is seemingly weakening, but this is not the first time that a Cyrodiilic Empire, lacking a Dragonborn Emperor, was on the verge of crumbling. Potentate Versidue Shaie proved that just because an empire was on the decline didn't mean that the trend couldn't be reversed.
Versidue Shaie was also an Akaviri Tsaesci, a Vampiric Serpentman likely centuries old. The Tsaesci are pretty legendary for their tactics, magic, and brutality. The Tsaesci are the reason why Black Dragons went extinct in Akavir, and they're the ones that crushed the Empire when it tried to invade Akavir. They are well known in lore as military genuises. The "current" Emperor, Titus Mede II, is a damn fop. An entire boat of Penitus Oculatus soldiers couldn't even protect him from a single Dark Brotherhood Assassin. Do you think they could've kept his ass alive against all the Thalmor Spies/Assassins that are in the Imperial Court?
Like i said before in other topics, the Empire is doomed. The very moment the Emperor issues the order to attack the Aldmeri Dominion, the Thalmor will know about it before the order even reaches the Empire's troops. They will know when/where/how the attack will happen, and can just completely [censored] block the Empire with a ambush. Hell, they could even intercept and alter the orders, so the Empire ends up sending it's troops to the middle of nowhere while the Thalmor sack the Imperial City again. It'd be the Empire's fault too, for signing that damn White Gold Concordant and letting the Thalmor in so completely.
The Thalmor would likely have a much harder time planting spies in a Stormcloak unified Skyrim, because of Ulfric's supposed "racism". If Skyrim and Hammerfall unified against the Thalmor, it'd be much more of a threat to the Thalmor than the Empire is.