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.
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.