The DLC is specifically tailored for those who played the main game and want more of a challenge. The vampires are pretty easy to kill, it is easy to work out where they will spawn and what you do to cause it. But you ignore that or cannot be bothered taking precautions, you will end up with a harder game since you will lose your NPC's. I doubt those doing all the moaning got any more attacks than those who never had a problem, they just failed for whatever reason to do anything to prevent it or deal with it.
My point exactly. You can choose to deal with the vampire attacks via approaching game play in a very narrow, specific way, through "work[ing] out where they will spawn and what you do to cause it" and then, well, endlessly babysitting your NPCs every single time you go near a town. If you do not choose to drastically alter your play style and become an endlessly spawn-monitoring NPC babysitter, then "you will end up with a harder game since you will lose your NPC's."
I disagree that this new dynamic makes the game any "more of a challenge;" as you note, the "vampires are pretty easy to kill" regardless of difficulty setting. What is challenging is keeping them from one-shotting your pathetically weak townspeople.... again something that is not affected by difficulty setting (since difficulty setting only affects the player, not the NPCs). You can always just let your NPCs slowly die off, but this doesn't really make the game any "harder" --- just a lot more lifeless. But you
have to choose one or the other..... or, you can, as you say, always choose to not play the DLC.
Somehow I think that the designers didn't quite intend for things to be that way, because if they had, they'd probably have done a lot more (or indeed, anything at all) to highlight the "hardcoe" or "challenging" nature of the DLC, or make it so that Dawnguard could only be started once you were high level and all finished with the non-essential NPCs. Instead, they allow you to start the questline at level 10, and do absolutely nothing to highlight the profound, world-altering danger that the vampire threat presents---they just use the same exact vague language about a "great threat" that they used for dragons and the civil war (neither of which turn out to be particularly threatening to your game world; yes, dragons can kill NPCs too, but as countless people have pointed out, the big difference there is that it's possible to
notice that a dragon is killing your NPCs without having to learn, memorize, and repeatedly check spawn points throughout a town every single time you enter or exit a building).