"Police bad" doesn't account for differences in performance between SF police and others. "DA bad" at least tries to do this (although as you note, is in some tension with the facts).
Sure, DA may not be the major issue on a fully objective level. But his point that "police suck" does not explain why SFPD is so bad relative to other cities still stands. If it's purely a police force problem, what makes SF police different? Genuinely curious if anyone has theories about this besides DA.