This, indeed, is a major step for Waymo. While some people may be skeptical about the safety of these rides, we all saw this coming. This has been a part of the ambitious goal most self-driving companies set out when they start. The reason I say "part of the goal", is because the end goal is L5 - to handle more complex scenarios which I assume Waymo is still working on.
Taking the ninety-ninety rule or the rule of credibility into consideration, the remaining 10% of development will probably take 90% of the time. By this, I mean that reaching a stage where L5 cars are owned by consumers can take a long time (if at all people want to buy instead of sharing, but that's a separate topic). For example, running this project in Pittsburgh, Boston, or SF is much more difficult than in suburbs of Arizona. Conditions with rain, snow, uneven terrain, high population density are still difficult to handle.
Several challenges still remain, from both technical and legal perspectives. However, any of that may not undermine the tremendous opportunities for Waymo (or its partners). There are several areas/markets that are ready for this technology (e.g., ride sharing in suburbs, transportation of non-dangerous goods over passengers, etc.)