So, the argument is that the vampires attacking towns are overcome with bloodlust and have lost control of their senses and therefore behaving irrationally? Hmm. I see some issues with that explanation . . .
First of all, I would reject the premise that the majority of vampires are little more than animals, as this does not seem supported by the in game evidence. There are over two dozen named vampires in Skyrim that fall into the Harkon, Hert and Hern category of being able to function in an intelligent and cunning way. There were not as many named vampires in Oblivion but in addition to Janus Hasildor, there was his wife Rona as well as Seridor, who was able to blend in with townfolk unnoticed until someone witnessed his feeding and he had to orchestrate an elaborate plot to cover that up. Even the vampires that we encounter in various caves in both Oblivion and Skyrim who are hostile, often seem capable of intelligence and cunning based on the activities they seem to be performing before we disturb them, which include things like enchanting things with black soul gems and doing basically the same types of things that we see necromancers doing. And in Morrowind, there were various vampire factions with questlines and whatnot.
To be sure, some vampires can be overcome with blood lust, like the vampires in Oblivion's Azura quest that were locked away for an extended period with no chance to feed, but I don't see a lot of in game basis to conclude that the majority of vampires end up incapable of rational thought.
But, even assuming for the sake of argument, the truth of the premise that the majority of vampires in the world are unable to control the bloodlust long enough to form rational thoughts, it still does not explain the connection between the random attacks and Harkon's plot that the Dawnguard questline is centered around. Random irrational vampires lusting for blood and attacking the nearest warm body seems totally unrelated to Harkon's plans
So if uncontrollable bloodlust is the explanation for the attacks, and that is unrelated to Harkon's plans, then we are left with not one, but two totally unrelated "vampire menaces" to deal with and defeating Harkon should not stop the random attacks from happening. If bloodlust is the reason for the vampire attacks, then the attacks should continue until the PC has cleared out all the vampire caves in the game.