I completely agree with your assessment of Kerr, and there is nothing wrong with purely legal analysis.
What got to me initially was the deliberate limiting of legal scope to warranted privacy violations, as though warrantless privacy violations are insignificant. He's welcome to do so, but he should be clearer about this and less rhetorically dedicated to his agenda.
EDIT: I see now that he reasons this technology only affects warranted privacy violations, since Apple has a policy of refusing unwarranted inquiries. He is overlooking the possibility that Apple may nonetheless be pressured, deceived, or compelled to violate users' privacy. He doesn't realize that Apple's claim of being invulnerable to warrants also protects the user from Apple's own vulnerabilities. It's not just USA law enforcement that targets users by exploiting Apple's access to protected data.
My understanding is that warrantless privacy violations have grown tremendously since the Patriot Act, both in scope and quantity -- that we have many new laws or codes since then which reclassify previously warrantful privacy violations as now warrantless.
He asks "How is the public interest served by a policy that only thwarts lawful search warrants?"
This is a nasty rhetorical trick. Does Apple's policy change serve any other purpose than to thwart lawful search warrants? Kerr puts "no" in your head, but of course the answer is yes.
He then makes appeal to the "civil libertarian tradition" of protection from warrants, saying that Apple is thumbing its nose at "that great tradition". What a joke. Apple is selling a product with powerful capabilities, some of which can be used to break the law, all of which have legitimate lawful purposes. Are Ford, Boeing, Delta, Budweiser, Louisville Slugger, Smith & Wesson all playing dangerous games as well?
Lastly, this is about removing Apple's ability to unlock. Just like Colin Percival's tarsnap. Doing proper security via proper cryptography. Minimizing untrusted third parties. Apple is getting criticized by Kerr for doing the right thing for its users. There are no victims in this action. It should matter not a whit that this makes the government work harder to violate privacy. Apple has no general duty to offer permanent taps for the government's pleasure.
This is why focusing only on warrantful privacy violations is a travesty of justice.
Please watch this before re-reading Kerr: https://www.youtube.com/watch?v=eT2fQu50sMs