And my understanding is that everything after the 5C is less vulnerable to even this attack (Which itself may not be possible, even with a firmware update.)
From what I heard later revisions won't accept a firmware update without either providing the passcode, or erasing the private key.
Arguably, the fact that the 5C accepts a firmware update without the passcode is a security vulnerability and ought to be patched.
The real problem is that you don’t want to set any precedent at all. Once it’s possible to do something for the 5C, weasel words can be introduced to make claims like “well: now you must maintain the current level of access by law enforcement”. Next thing you know, that excuse can be used to interfere with all future hardware designs.