I don't think it is defeatist.
The whole point of security is risk management. You address your most common risks first, and then the less common risks later. Electrical fires are more common than hijackings, especially after you figure in airport security. Therefore in plane design you worry about electrical fires far more than you do hijackings.
Additionally this is the first incident of its kind. This is not a typical hijacking. In fact it is entirely unprecedented. For this reason it is a game changer, and I think you are right to point that out. But electrical fires are orders of magnitude more common so you aren't going to ever risk one based on a much less likely possibility here.
To be honest, I have faith that investigators will eventually track the culprits down. Then once we know who and why we can decide what to do about it. However consider what is required to pull this off:
1. Having an airport capable of landing a 777 that nobody knows about
2. Having refuling equipment and fuel.
3. Having a hangar capable of storing the 777 out of the eyes of satellites.
These are not small requirements. Whoever did this was quite prepared. It was not pilot suicide based on the data we have. Whoever did this was large, organized, and had tons of resources (we are talking about a large drug ring or a small country here). Against a determined enemy like that I don't know that you really can stop them at the plane. Instead you need real defence in depth, and we can't even talk about that until we know more.