I'd argue it's the only framework which can be applied where endings are concerned.
Not entirely sure I understand this. It seems to mean "what is moral is whatever the PCs decide is moral" and that's...I mean technically
it's a moral standard, in the sense that a refusal to choose at all is still a choice. But it's a null choice and I don't really see that as a "framework" in any meaningful sense.
No amount of +++ will prevent criticism of a nonsensical premise though. And it cannot even be addressed until you explain how such endings can be enforced outside of player actions.
Because a prewritten adventure is authored, just as a book or film is authored. A book can have a message or inherent moral compass even if individual characters do not act in accordance with that message or compass; generally, this will result in those characters becoming (or staying) unhappy or being punished (whether in a very practical way e.g. legal consequences or in a more symbolic way e.g. enduring preventable suffering.)
Vader turns evil, and suffers for it. His return to good requires a heroic sacrifice, which kills him, but the act allows him to obtain some measure of absolution, almost totally separate from Luke's own actions. (Heck, Luke briefly does
embrace the dark side and then stops himself.)
In other words you need a well defined moral framework in order to present a moral ending. And even then it assumes no dilemmas and depends on player choices.
I dunno. I think it's quite possible to have "you need to resolve
this dilemma in order to earn a happy ending" as a story element. It's a motive to induce people to Take A Third Option. Finding a way to save MJ and
the busload of orphans.
And sometimes it will fail. Returning to Vader, you could argue that Luke "failed" to resolve the dilemma of stopping the Death Star and
saving his father. He got the warm fuzzy consolation prize of his father redeeming himself, but not actually saving his life.
There's even an alternate timeline comic where Leia went up to the Death Star with
Luke, and things play out differently: they're able to save Anakin, but at the cost of failing
to kill the Emperor, thus allowing the civil war to continue for longer. That pretty clearly paints this as some kind of dilemma, of having to choose what victories are worth seeking and what you're willing to accept imperfect or symbolic victory on.
Finally, D&D often includes actual deities, sometimes ones that are genuinely transcendental moral paragons. If Bahamut is a transcendental being literally made
of pure Justice and Mercy and Goodness, then him telling you something is morally wrong is...kind of hard to argue with, within the premise of the story. Either you must reject that the story is what it claims to be, or you must somehow argue with (effectively) Goodness Itself embodied and conversant.