We are chasing the 9s... diminishing returns are still returns. If 10x the data improves from 99.999 to 99.9995, that is still progress. Maybe 100x data gets you to 99.9999.
Are we even at 80% yet, compared to non-drunk/drugged/sleep-deprived regular human drivers? All of the videos I've seen are well below the driving capacity of a human taking their first driving lesson.