No system is completely immune to a determined attack. Network security is reactionary by nature. You cannot protect yourself from an undiscovered exploit, and there is always an exploit to be found. As your systems evolve in an attempt to improve them, new flaws will present themselves. Sony has not been sitting idly by, oblivious to the potential for another attack, they're inevitable. Look online for even a moment and you will find that most organizations have been hit over the last few years. Goverment, banks, hospitals, software developers, anybody and everybody. It's part of living in a connected world.