It seems like Uber put in this "reaction delay" to prevent the cars from driving/maneuvering erratically (think excessive braking and avoidance turning). This, along with allowing the cars to drive on public roads at all before handling obvious concerns like pedestrians outside of crosswalks, is supposed to be balanced out by having a human ready to intervene and handle these situations.

I think one of the biggest lessons here is about the difficulty of relying on humans to maintain attention while covering an autonomous vehicle. Yes, this particular driver was actively negligent by apparently watching videos when they should have been monitoring the road. But even a more earnest driver could easily space out after long hours of uneventful driving with no "events" or inputs. And that could be enough that their delay in taking over could lead to the worst.

Certainly not defending the safety driver here - or Uber. But I think there's a bit of a paradox in that the better an AV system performs, and the more the human driver trusts it, the easier it is for that human to mentally disengage. Even if only subconsciously. This seems like a difficult problem to overcome, especially if AV development is counting on tracking driver interventions to further train the models for new, unexpected, or fringe driving situations.