There is nothing to fix, they will not do anything about it. They already gave the player plenty of options to get through a DLC challenge you chose to play, if your current character is not up to the challenge, it is up to you to adjust how you play to get the results you want. For keeping NPC's alive you would either replay your failures, till you got it right. or by avoiding the attacks completely and not going to any towns while doing the main Dawnguard questline. Or you can just uninstall the DLC if you do not want to play it. The developer having already given a way to avoid the town attacks, is not going to pay extra money to patch the DLC just to make it even easier.
That is absolutely not true, there is plenty to fix. Shopkeepers getting killed are the worst because they have finite money. It's not like in Oblivion where if the Three Brothers shop and Jensine went down, it wouldn't be the end of the world due to the other shops and infinite merchant money. With Skyrim if that happened especially in Whiterun where Adrienne gets killed pretty much every time, it's devestating. If your not vigilent, the only merchants that will be left are the Thieves Guild ones, Babette with the DB and The College Of Winterhold. The biggest problem with that is everytime I'll need to check the city to make sure that Vamps aren't attacking, that will get old real fast.
It is easily the worst design decision besides the use of the Dragon Scroll IMO, that Beth came up with for this DLC, it's a feature that was great in theory but horrific in execution.
This basically forces you to play Dawnguard instead of making it optional, we have never had that in a Beth game before having to be forced to play something that should clearly be optional. Sheogorath never forced me to play Shivering Isles or gave me visions every midnight, I never had to go to Solsteim in Morrowind, or Point Lookout in Fallout 3, etc. With Skyrim though, I have to do Dawnguard otherwise I'm going to be out of essential citizens eventually and no I'm not going to alter my gameplay just for the sake of keeping the NPC's alive, I shouldn't need to do that in the 1st place. That will get repetitive very quickly.
Until Beth fixes this feature which they probably won't, I'm not touching Dawnguard again. The easiest way to fix this feature is to only have the Vamps start attacking after you exit the Dawnguard Fortress the 1st time. Sadly I doubt that's going to happen.