Something that confuses me about this whole "the entirety of Tamriel will rise up and vanquish the Vampire Lords if Harkon does this," train of thought so many people are going down is it assumes too many things:
*variables 1 through 7*
And these two are major ones to me
8) The Vampire Lords would just sit at Castle Volkihar and twiddle their thumbs and toes while they wait to be besieged.
9) After traveling for days/weeks/possibly months in total blackness, the various groups of soldiers would be in well enough mental health to be able to cope with prolonged battles in the dark against enemies who can see perfectly fine in total blackness, have superhuman strength and reflexes, can disappear, fuel themselves with blood, can revive their recently killed friends and force them to fight one another, summon gargoyles, turn into bats, throw them incredible distances, paralyze them, and can transform into winged monstrosities without panicking and routing.
To me, this seems like way too many variables for the humans/mer/beast races to be able to effectively control and manage. There's no way that this event would be enough to unite them to work together to save themselves. Even when it was only two factions, they couldn't even agree to work together, and that was when the World Eater returned. The most they agreed to do was grumble in their respective cities, waiting for things to sort themselves out. I can't imagine this being much different, and you're looking at organizing entire countries with vary different political aims. Who's to say the population of Valenwood that wants to be free of Thalmor control wouldn't just start killing them in the confusion, rather than working towards fighting the Vampire Lords? Same with the Stormcloaks. Solitude is closest to Castle Volkihar, so the Stormcloak soldiers could seemingly march towards the vampires, but then attack the city and take Jarl Elisif hostage and force her to kneel to Ulfric.
There's too many variables for the mortals, which I think really pushes things in the vampires' favor.
Thank you, thank you, thank you. It's like you reached into my head and read my thoughts!

In short: I just don't see it. I'm not saying that the Vamps would win, hands down. It would be a world changing event for certain, but I just don't see how Skyrim, let alone all of Tamriel, would be able to respond quick enough to the threat. As REL_D pointed out, there are too many variables. So many that the deck is really stacked in Harkon's favor....not to mention in the favor of all the other Vampire Clans out there.
