Let me explain this as simply as possible. Imagine I build a whole neighborhood full of houses with no windows and each house has only a single door. Imagine also, the people who buy these houses from me put their own lock on their door and keep their keys with them 24/7. Those houses are as secure as they can possibly be.
Now imagine the same scenario but I, the builder, put a back door in everyone of those houses and I have a single key that will give me access into any of those houses. The mere existence of that 2nd door undermines the security of every one of those houses.
What if an employee of mine steals the key either because they want to go snooping through the homes, or they want to make money selling the key to a criminal or a criminal threatens / blackmails them into stealing the key? What if a criminal hires the best locksmiths he can find to create his own key to the backdoor? It wouldn't make sense to spend that much effort to make a single key to get into a single house but if he can make a key that fits the backdoor, he can get into any house in the neighborhood so the reward is much greater and he would be willing to put a lot more effort into doing it.
This is what I (and several other people on this forum and Apple themselves) want to avoid by not having the backdoor in the first place. If the NSA itself can't keep their "cyber arsenal" secure by the means you described above (i.e. stored in an offline computer) why should anyone believe that Apple could keep the key to the backdoor secure?
I am sure you have nothing to hide in your home. But just to set a good example for the rest of us, why don't you mail a spare key to your home to Officer Mohamed Noor at the Minneapolis, MN police department? That way you can "sleep at night" knowing the only vulnerability to your home is a law enforcement officer "and we can continue to catch the criminals". Just one piece of advice, if Officer Noor ever uses the key, don't make any sudden moves, he apparently startles easily.
http://www.dailymail.co.uk/news/art...d-shooting-Fake-signs-posted-Minneapolis.html