There is no doubt in my mind that if a hacker really wanted to he'd be able to get into the Live network as well.
The best proof that this kind of thinking is wrong is that the Live network, like so many others, has not been hacked. The same incentive is there and you can guarantee that it is a massively desirable target.
But if that were
true, it kinda hurts the case for Sony more. As I said before, the real crisis of this situation is not that Sony got attacked, but that they didn't sufficiently protect their customer's records. It's super basic security practice that you don't keep that kind of information unencrypted, or passwords unhashed. So if you're assuming that it's impossible to prevent intrusions, you have double reason to follow that practice.
Again, I certainly don't condone the actions of malicious hackers, I'm taking it for granted
that they are to be frowned upon. But computer security is something that, unfortunately, is not always taken seriously enough, and when that negligence affects 77 million people, I don't believe they should be given a free pass.