I'm not so sure the Saints failed their first round picks for Oline and Dline. Theres a lot that goes into players being successful in the NFL. Constantly we see great talent look bad, go to a different coach, and suddenly their career is revived.
We know DA's coaching staff was bad and we know in some specific cases they weren't even attempting to coach up players. With the Oline, they looked lost at times. Out of sync. It's quite possible the scheme and play calling were failing them to some degree on top of coaching failing to coach them up.
Point is, I don't think we have any idea what our players in the trenches are truly capable of because coaching has been clearly failing them since Payton left. This was to be somewhat expected for anyone who watched the Oakland Raiders to a fair degree while DA was coaching there.