Personally, I feel better with a main dam being constructed of concrete. But, then there was the Francis Dam failure that had poor safety factor ratings and a faulty abutment zone (unknown geologic feature/structure).
The issue with the Oroville spillway was that the original design was flawed (the BOC report identified nearly everything we've uncovered in this long thread). I believe that DWR did not recognize clear sign of serious issues on the spillway. They should have raised the alarm on these issues and then realized they needed to replace the main spillway to modern standards.
But I firmly believe the design faced a potentially fatal worst case probable flood issue in needing to meet a 620,000 cfs then 646,000 cfs combined flow capacity. In DWR deciding to break the design into two structures has formed their destiny where they do not have many options today. Sometimes in projects you must recognize a significant design challenge (620,000 cfs flow hydraulics testing huge standing wave problem) and power through the problem even if it is hard politically, logistically, and engineering wise. There are no excuses in this high stakes engineering strata of dam designing.
Intuitively, so do I.