The closet thing for justification of the expense of an SLS (not Orion though) is the Europa Clipper and Europa Lander because of truncated mission time that SLS offers these programs. Such complex missions and a standing army of experts to support the mission causes a several year decrease in mission time does two things. Decrease the after launch mission expenses to almost in half and increases mission success because time in space for a probe is the biggest threat to success. A tremendous shortened mission time means a tremendous boost in probable mission success. At the moment the DV that SLS can offer these probe missions can not be duplicated by other LVs existing or soon to exist that can be provided by SLS within the timeframe for EC
#1 on my list of what it would take for me to support SLS would be for it to actually be designed for a reasonable flight rate. It is currently working on adding more evidence to the pile that a low flight rate will lead to a horrendously expensive rocket.That said, I wouldn't actually advocate for cancelling it for another couple years, when Vulcan/New Glenn/mini-ITS are far enough along to have more confidence in them actually flying. This is not me supporting SLS, but acknowledging that my expectations about these rockets could be wrong so I can understand why some support SLS for now.I would also like to see them skip to Block 1B for EM-1. Even with what has already been done, this seems like it would still likely be a cheaper and faster method of getting SLS running, though really it should have been done when they first decided not to do EM-2 on Block 1. As a bonus, the delay would mean it wouldn't fly before we get to the point where I expect the commercial capabilities to be far enough along, which would hopefully make cancellation easier.
By and large, we even behave...
I come down on the side against SLS. Not because I think the rocket is flawed ( I admire it technically), or too expensive ( which it is), but because I see SLS as part of an overall problem that prioritizes symbolism vs. actual accomplishment.
One think that might change my mind is if they replace the solids with reusable kerolox boosters. Have them land back at the cape to refuel and fly again. Also, if they use a shorter reusable kerolox core, and add a second stage. I believe costs would come down with a mostly reusable boosters and core. Payloads would go up at the same time. Existing rocket is an expensive kludge with todays technology.