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.