It even seems like Comey is aware that the particular technique he is asking Apple to use will not apply to future phones made by Apple, and that the reality is that cryptography will soon reach the point where the FBI will not be able to rely on decrypting data as part of their investigative approach.
Yes, if Apple creates this exploit, then that exploit will potentially be available to other state actors and criminal enterprises. But only for iPhones older than the 5s. Fundamentally, as other commenters have pointed out, this is not creating a backdoor -- this is using an existing backdoor to install a bigger backdoor. It is, of course, possible that in the future even newer iPhones might find themselves vulnerable to unauthorized decryption, but really, the FBI's ask in this particular case really is narrow in scope, because it does only apply to older phones.
I don't see the slippery slope here that many people seem to think exists. The slope begins and ends with a phone released in 2013. All any person (criminal or otherwise) has to do is buy a newer iPhone, and then this whole discussion no longer applies.