While dam designs can certainly function for a very long time with regular maintenance, this particular dam’s primary spillway was deficient. While we won’t know for sure till the spillway is forensically tested, the issue appears to be a failure due to cavitation. Maintaining a consistent spillway surface would help with cavitation, but it would never fix the problem completely. Putting in a stepped spillway isn’t necessary the best choice either. A better fix is putting air into the spillway through venturi action or otherwise. The air prevents the cavitation from being destructive.
As far as concrete slabs on subgrade are concerned though, rebar is a waste of steel. Highways don’t contain rebar for this reason. The only reinforcement they contain is slip pins between slabs to prevent the slabs from slipping out of alignment vertically with each other while still allowing the roadway to expand and contract. The failure of the spillway would have been slightly different, but it still would have failed progressively.
As far as the emergency spillway design is concerned, it performed as designed. It was never designed to be used for any extended period of time, and in fact this is the first time it has been used since 1968 when the dam opened. It could have been more robust to reduce the erosion near the weir, but ultimately it performed as it was designed.