Is it? Because while I think we can all agree that it borrows heavily from IX, I'd hardly call it an adaptation. It just borrows some names and figures, has a final zone deliberately evoking Memoria, and characters clearly riffing on Garnet and Steiner. I'd hardly say Dawntrail resembles FFIX in more than trappings at all, any more than I'd say Stormblood resembles XII. Hell, Sphene and Otis are more dark mirrors of Garnet and Sphene than anything. And I think the deliberate evocation of IX also serves a thematic purpose, to give what it would call Alexandria a nostalgic feeling to us as the player; Living Memory as a concept begs for the world it's evoking to feel familiar and friendly (contrast with Fake Amaurot needing to feel foreign yet strangely tranquil, and Ultima Thule steeping itself in the alien and hostile). FFIX is actually a perfect fit for that, as it's not only a nostalgic title by itself, but its choice of aesthetic was always very 'storybook', a whimsical and friendly depiction of a classic high-fantasy setting; very 'Disney'. I can't speak for the people who haven't played it, just as I assume you can't, but I feel like there's nothing else that could've managed it.