But the new SRB's and engines are expendable
They figured out how to expend them. Probably at increased cost...
Strangely, at the SLS's lower flight rate, Thiokol and Rocketdyne might not even have to increase their production rate of building brand new engines.
Early in the development of the Shuttle they considered putting the SSME's under the external tank (the aeronatical engineers were saying it was very advantageous, especially early in the flight, since the thrust vectors would be more aligned with the center of mass). But they still wanted to re-use the expensive engines, so they explained that after main engine shutdown the engines needed to somehow swing over from the ET to the back end of the shuttle, allowing it to jettison only the ET. The mechanical engineers started screaming about how hard that would be, a design nightmare that would probably weigh a lot, never work well, and where any failure (engines stuck halfway) would definitely result in the loss of vehicle and crew.
And that was the end of that.
It seems to me a more sensible option would've been to mount the engines on their own re-entry capsule (heatshield forward like a firewall) and have the engines re-enter shortly after ET seperation, probably after a partial orbit so they could be fished out of the Pacific near Rocketdyne's plant.
That would create advantages with turn-around time, because while the Shuttle is in orbit for a week or two, the engines are already being refurbished in California, while a prior set of refurbished engines is being mated to an ET for the next launch. It seperates engine inspection and replacement from the Shuttle post-flight refurbishment cycle. It also saves all the cross-plumbing to the Shuttle, and also cleans up its entire back end, lowering the drag significantly, while saving significant orbital and re-entry weight. That would lower the required wing area and boost cross-range, or lower landing speeds. It also seperates the design of the orbiter component from the design of the first stage booster, allowing each to evolve or upgrade seperately. And of course it means that a main engine failure wouldn't rip through the back end of the Shuttle directly.