Your constant underestimation of their ability's is exactly why they're such a threat, Ulfric and most of the Jarls and other Nords and Tulius and the Imperials would underestimate the "stupid goblins" as well and it would come back and bite them. It also very likely it will.
Oblivion? Cyrodill devastated within decades following events of the game
Morrowind? Morrowind devastated within decades following the events of the game
Skyrim? I'm sure you can guess where this is going if they continue the trend established with the last two games.
Lets see the possible events that could befall Skyrim in the future
Falmer attacks.
Thalmor Invasion.
Prolonged civil war between internal nordic factions if they successfully drive Cyrodill out.
Great war & it's consequences.
Unforeseen consequences of defeating Alduin.
Future events set by Arcanos messing with they Eye of Magnus.
Whatever they decide to do with the Dragonborn.
Further prolonged conflicts with Neighbouring provinces.
If Paarthurnax survives the events of the game, perhaps the blades where right and him and the remaining dragons manage to reform the Dragon cult or do something else to cause chaos.
Unlikely but still plausible Akavir invades Tamriel again.
Whatever caused Atmora to freeze over and drive the Atmorans out in the first place starts to effect Skyrim and northern Tamriel.
Something Relating to Snow-throat tower, whether the tower is the Throat of the world or something else.
Hermaeus Mora gets bored
Though some of those are less likely than others, Id rank the Falmer causing trouble as the most likely and the one with the most in game evidence to back it up. But it could be something entirely different that Bethesda comes up with. But I'm certain enough that skyrim is not going to come out of following events Unscathed that I would bet money on it.