My gut feeling about the "SLS - Orion" insanity is that NASA has long understand, Congress will never allow for cancellation.
Thanks to that coming book by Lori Garver, we have some hindsight about the Constellation > SLS-Orion transition.
Congress more or less blackmailed Garver and Bolden - basically "keep the Shuttle pork barrel alive, SLS & Orion, otherwise, we will start screwing science, aeronautics, satellites - whatever is for grab in your budget."
NASA is proceeding with SLS-Orion, despite cost overruns, Lockheed, lack of funding and other plagues
- because Congress told them to do it in 2010
- because it is presently integrated into Artemis.
But make no mistake... I'm quite sure they secretely want that monstrosity to die. For obvious reasons. And they have a plan for that - to screw Congress.
One astonishing recent development was SpaceX "Moon Starship" being enlisted into Artemis.
To me, there is little doubt that NASA has a hidden agenda over this.
Now that they have SpaceX and Moon Starship onboard the Artemis train, they are patiently waiting for the system to prove itself. Within three or four years it will be done.
Guess what will happen then ?
NASA will quietly make a cost/efficiency study - something akin to "SLS-Orion vs Moon Starship for crew transfer to Artemis lunar base".
I think we already now the result of such study... NASA probably already has some idea about this, too.
To me, they will pass the said study to Congress and hopefully, it will the death blow to SLS-Orion.
I mean, for the exact same mission "let's transfer crews to cislunar space" Moon Starship completely flattens SLS-Orion like a steamroller - 100 times less expensive, and far more than Orion 4 astronauts.
Let's say these two systems, by 2026, are used for the same mission. I really can't see how could NASA justify, even for a split second, SLS-Orion against Moon Starship.
Not only it will cost $2 billion per flight; but NASA has NOT been given these billions, in its budget. Not since freakkin' 1966 !
We all know what is needed for Constellation (or SLS-Orion, same animal, different name) since 2009 - Augustine crunched the numbers quite well.
Basically, NASA will struggle to fly that thing once every two years when Moon Starship will acumulate plenty of flights during the same period - 10 or 100 times less expensive, and from a different pool of money.
Frack, it would be very much Hughes Spruce Goose vs Boeing 747. Same size (give or take) and overall shape but... it stops right there. Particularly for economical passenger transportation across the Atlantic.
Incidentally, the above says how much desperate NASA is to get ride of that last relic of Shuttle / Constellation era with Congress not allowing for it.
Since they can't kill the monster outright, they let it agonize for 15 years (2010 - 2025) - or even 21 years if Constellation is included.
Frack.
