Oh come on, surely you don't think I'm absolving the criminals of their actions. That's completely beside the point.
It's pretty impossible to quantify how secure a "system" is (to use necessarily vague terminology), especially since there are so many different types of security involved, but it's not as you say. The point is that this
attack was completely avoidable, a simple oversight that allowed the attackers to bypass the real security. But the thing that makes it more akin to negligence is that such a compromise allowed access to PSN user's sensitive personal information. That should never happen.
Remember, the attack itself has not caused any down time and would have been pretty harmless if Sony had properly protected that information. They took the network down themselves because they can't run it until they do things "right" in order to, erm, minimize the class action lawsuits.