https://lawfareblog.com/not-slippery-slope-jump-cliff
Quotation from, Not a Slippery Slope, but a Jump off the Cliff By Nicholas Weaver.
The request to Apple is accurately paraphrased as "Create malcode designed to subvert security protections, with additional forensic protections, customized for a particular target's phone, cryptographically sign that malcode so the target's phone accepts it as legitimate, and run that customized version through the update mechanism". (I speak of malcode in the technical sense of "code designed to subvert a security protection or compromise the device", not in intent.)
The same logic behind what the FBI seeks could just as easily apply to a mandate forcing Microsoft, Google, Apple, and others to push malicious code to a device through automatic updates when the device isn't yet in law enforcement's hand. So the precedent the FBI seeks doesn't represent just "create and install malcode for this device in Law Enforcement possession" but rather "create and install malcode for this device".
Let us assume that the FBI wins in court and gains this precedent. This does indeed solve the "going dark" problem as now the FBI can go to Apple, Cisco, Microsoft, or Google with a warrant and say "push out an update to this target". Once the target's device starts running the FBI's update then encryption no longer matters, even the much stronger security present in the latest Apple devices. So as long as the FBI identifies the target's devices before arrest there is no problem with any encryption. But at what cost?