Just having the key doesn't automatically make it easier for you to create the new IOS. That task is significantly easier for Apple to do than for you to do. Much of security is raising level of effort for an exploit to a high enough level that it's not worth the investment required to create it.
If Apple creates the new IOS for you though and all you have to do is get your hands on it then the investment required just got smaller. Any reduction in the investment required for an exploit should be very carefully evaluated. Saying things like "This can't possibly be used by someone else." is either disingenuous or signifies that you don't adequately understand the issues, or you are lying to yourself. Whichever one of those that it is. It's in Apple's and frankly my best interest that someone highlight the truth of the situation.
[EDIT]: The below is more hyperbolic than I intended after a second reading. I'm leaving it since I already hit submit but I wanted to apologize for going there. :-(
This is like saying
Look... We'll create a nuclear bomb but it's okay we're
the only ones who will have it and nobody else is going
to be able to get one.
Maybe you can guarantee that no one else get's enough information from you to build one. However once you've built one the chance that someone could get the information from someone involved in your project to accelerate their creation of a nuclear bomb just increased by a non-trivial amount. And the potential risks are really high. Pretending otherwise would be foolish.
My claim is that the resources required to steal Apple's keys are significantly higher than the resources required to modify iOS in the way required. Since an successful exploit requires both modified iOS and the keys, if I am correct then creating the modified version does not significantly decrease the resources required to exploit.
When I claim it can't be used by anyone else, I mean without Apple's signing keys. There are no shortage of jailbreak developers that would be happy to work for whoever pays them, and could build tweaks in to iOS with the signing keys.
For the government to make the necessary modified version of iOS, they need to have or acquire a team of skilled engineers that either already have detailed knowledge of iOS and the devices it runs on, or a way for their team to learn that knowledge. This would probably be somewhat expensive in both money and time.
To acquire Apple's keys, the government only needs someone who can replace the device key and re-sign a new iOS package, and a sympathetic judge that will sign a subpoena, warrant, or national security letter. This is practically free and shouldn't take more than a day or two (warrants are sometimes granted near-realtime).
I don't understand why would think taking Apple's keys (which wouldn't require "stealing") somehow requires more resources? A warrant or nsl is much cheaper than a team of developers.
You make this sound hard: there are tons of qualified people who could do this in less than a week, including myself. We already have all of these tools just sitting around from the iPhone 4, and some of us have emulators for more recent devices: the only thing we don't have is Apple's key.
If Apple creates the new IOS for you though and all you have to do is get your hands on it then the investment required just got smaller. Any reduction in the investment required for an exploit should be very carefully evaluated. Saying things like "This can't possibly be used by someone else." is either disingenuous or signifies that you don't adequately understand the issues, or you are lying to yourself. Whichever one of those that it is. It's in Apple's and frankly my best interest that someone highlight the truth of the situation.
[EDIT]: The below is more hyperbolic than I intended after a second reading. I'm leaving it since I already hit submit but I wanted to apologize for going there. :-(
This is like saying
Maybe you can guarantee that no one else get's enough information from you to build one. However once you've built one the chance that someone could get the information from someone involved in your project to accelerate their creation of a nuclear bomb just increased by a non-trivial amount. And the potential risks are really high. Pretending otherwise would be foolish.