Well, that's where Constellation dropped the bag and SLS picked it back up, despite having several other systemic and structural issues with the program. The Saturn family was a "jack of all trades" project: it was designed from the ground up to be modular and have a configurable stack, to meet a variety of mission portfolio demands. In practice it only boiled down to two configurations, the I/IB which was just an S-I/IB and an S-IVB, and the V which was an S-IC, S-II, and S-IVB.The big lesson NASA had learned (and the DoD still hasn't judging from their stupid fighter jet) is that when you're dropping this kind of money just make specialized tools for the jobs you need, consolidating into a jack of all trades project is a bad idea.