I'm not sure the issues are separable in the way you'd like them to be. The fact it creates a precedent makes what ever software they write a "de facto" back door, as Apple will either have to keep the software, or be faced with writing it again whenever the government asks them too, if this order is allowed to stand.
It is not in dispute that Apple can write the software, what is in dispute is whether they should be compelled to write it, to effectively co-opt Apple into government service, to enlist the employees of Apple, perhaps against their will, to work for the government. This can not be allowed to stand, it sets a terrible precedent that when combined with Apples ability to write the software gives the US government, virtually unlimited power to spy on us. If Apple can be compelled to write software to bypass the lock screen, they can most likely be compelled to write software to remotely turn on the camera or the microphone, after all, if they don't, won't they be helping the terrorists. This for us or against us dialogue the government is using is insidious in the extreme. No issue is so simple that it can be thought of or reduced to such simple terms.
As for others gaining entry, once the system is weakened it gives others a way in, even if it can't be applied directly, it can be decompiled and give hints about how to approach the problem. Getting hold of it will not be as difficult as you imagine either. No server is completely secure and even if it is kept on an air gapped machine, it will still not be invulnerable. In any case how long would it really be before the FBI turned round and said that in order for a more expeditious service they'd really rather it be kept on their systems.
Just because something can be done, it does not necessarily mean that it should be done.
