Are there any technical reasons for not simply continuing the space shuttle production, except for replacing the orbiter with a pretty simple cargo bay (a payload fairing)? Like the Russians planned in the 1980's with their Energia. It would seem like a cheap and immediately applicable thing to do, compared to the SLS concept which requires the expensive and decade long redesign of each and every component in this old system, such as the main engines, the boosters, the fuel tank, the launch pad, the mobile launch tower, the barge, the welder, the test stand and on and on.
It would immediately seem much cheaper, safer, faster and simpler. Redesigning reusable engines to become non-reusable doesn't sound like the future to me, but I'm maybe missing something essential here?