When the back door is created, it cannot be destroyed. FBI had tons of other phones they wanted to access [1]. Hacking one phone would set a precedent that Apple would have to hack numerous other phones.

It is so simple to steal software, just like NSA's hacking tools were stolen.

If NSA cannot keep software safe, why can you think that Apple can? I think it's unreasonable to require Apple to have a higher level of security than NSA effectively has.

1: http://www.sfgate.com/nation/article/Apple-FBI-face-off-in-C...