> How do we decide when it is negligence, and when there was nothing that could be done?
As others have pointed out, that question is one that courts deal with every day in other industries.
IANAL but I have some familiarity with the architecture and construction industry, where there are lots of lawsuits around negligence. My understanding is that the question is generally framed as "what would a reasonable professional have done in this case?" Would any reasonable contractor have interpreted those drawings to mean the joists should be spaced 24" apart? Or would a reasonable professional have interpreted it to be 20"?
The same standard could be applied to software engineering and data breaches. Would a reasonable engineer allow a SQL injection vulnerability to persist in 2016?
I suspect as soon as any amount of precedent is set for lawsuits around "hacking" law firms will push open the floodgates and it will suddenly become very common.
In the construction industry the frequency of lawsuits has created a culture where people go to great lengths to reduce their risk of liability. Architects draw deliberately vague details around waterproofing and contractors push to have every detail for how to do things spelled out on paper, so they won't be on the hook if one of the steps in the process is wrong.