From a UX (and potentially legal) standpoint, it matters less how many warnings he ignored in the few minutes prior to the crash, but how many warnings he—and others—ignored on other drives.
By way of extreme example, if every time anyone engaged autopilot, it issued a warning every minute, you would quickly ignore them. Information fatigue.
By way of extreme example, if every time anyone engaged autopilot, it issued a warning every minute, you would quickly ignore them. Information fatigue.