But the FBI's general counsel doesn’t write letters just for the hell of it. Something’s afoot.
They chose this case to try again for the same reason that they chose the San Bernardino case: scary Muslim foreigners killed Americans, so public sentiment will be on the FBI's side, in turn creating pressure on Apple to comply.
When Apple inevitably refuses, the Feds will float it as a test case, because their wet dream is a favorable Federal court ruling that establishes precedent forcing Apple to assist. Once that precedent is established, they can then use the case law to force cooperation from other companies that provide encrypted devices and/or comms. (Barring another 9/11 scale attack, it's a long shot that Congress would pass legislation to that effect IMHO.)
If the Feds read the tea leaves and think the case might produce the opposite result, they will back down (as they did in 2016) and pay some company such as Cellebrite to do the dirty for them. Their worst nightmare is a Supreme Court ruling that companies such as Apple do not have a duty to hack their own systems. By backing down from the demand if it looks like they might lose, they moot the case so that Apple (or whoever) no longer has standing to pursue it; thus preserving the current state of ambiguity.