Not so. If the DG already knows where the castle is (random attacks), then so does the rest of Skyrim and so would Tamriel.
Key word being "if".
As I understand things Isran has no idea where the castle is, nor do any of the other DG members. Granted, I'm only half way through my DG-side play-through...so I don't really know the specifics of how it ends...but so far I (the PC) am the only one that knows. This conversation is assuming that the PC sided with Harkon. Isran and the DG still don't know where it is...and besides, if you take your actions in the Vamp side quests into account then
Spoiler Isran and the DG are all dead. So all their special tachtics, armor, spells, trolls and whatever else don't really factor into this. At least not for a while. I would assume that eventually someone would search, find, and attempt to resurrect the order; but by then Harkon would be way ahead of that movement.
The vamps just aren't enough of a threat to claim all out war at the moment, but you can certainly bet one would squeal once put to the test after being captured (and townsfolk are killing them) and let the humans know from where the darkened sun originated. Not in a day maybe, but a week or month, sure. The only recourse, for the entire world, would be to descend upon Harkon's lair and gain the "cure", if there is one, or die trying. There is absolutely NO WAY a few VLs, pureblood or not, can stand against the onslaught of all of Skyrim, all of Tamriel, all of Nirn.
I refer you to the quote bellow

There is to much chaos going on for the people of Skyrim to even really worry about the vampires did you forget about the dragons. the empire and the stormcloaks probably wasted most of there resources fighting each other. plus the people of skyrim are not to keen on the use of magic and VLs uses magic. Don't forget that vampires can raise there numbers with undead minions and replenish them. I just believe that could take over in the misdt of all this chaos.
...moving along...
How can that not be understood? That's an important statement/message in the story. No matter how strong an individual is, he can not stand against everyone else. Would it be nice to teach you that lesson the hard way? Absolutely, but nothing p's off a player more than to stack them up against an unwinnable war to an unwinnable conclusion. Only a small army of 10,000 would be more than enough against 12 vamps, dragonborn or not. Do you REALLY think you could stand against that many? Try it. No god mode, no cheats, just console them in 10 or 20 at a time and see how many rounds you can go, then you can say you "joined Harkon".
I'm not arguing the story that was told. I understand the message. It was well told, and I enjoyed the adventure. Siding with Harkon is merely a third option that is worth at least exploring, and for it to fit the overall story would certainly need to be revised.
...an unwinnible war, mmhm, yes yes, I agree. That would anger a great many players. Bethesda is too smart to throw the PC into that situation without an exit for them. To do so would be...well...stupid for so many reasons that I'll not go over. Most of them involve
$Money.
But I think perhaps you're underestimating Harkon's intelligence, and that of the other Vamps in his court. It would take time and movement for an army of 10,000 to amass. As
Lord of Dragon mentioned, the Vampire Lords are exceptionally skilled at Necromancy. Bolstering their numbers wouldn't be an issue, maybe not to 10,000, but enough to make the army wade through something really nasty. [Edit: I can banter back and forth about tactics in later posts.

]
Point I'm trying to make is that it "could" be done. It's a possibility, if there were an Option #3 and Harkon had his way. I give you a

for your tenacious optimism.