DDO's attrition is the same reason it's always been. Gaming moved on with big strides and DDO made zero serious investment in future proofing its gameplay (which would have required overhauling AI). They squandered their strategic advantage (which honestly I think they lucked out with) with active combat which bought them more runway than the rest of the game features combined. DDO has stalled out as an essentially rpg system that was advanced for their day, but is now clung to by sunk cost fallacies. It like someone being addicted to Temple of Apashi (sic) and refusing to see the gold box games were better implementations.
Elden Ring just broke new ground with the future of active combat with smarter enemies (although even there, there can be big leaps forward with big data tools), open world, resource/crafting, and it only awaits the snap on component of formal MMO (it already has grouping in a limited sense).
I could care less where DDO goes anymore, as gaming is moving closer and closer to what I've wanted in gaming regardless: reactive enemies, open worlds, resource/crafting, and transitory grouping, all seamless, all integrated, all beautiful.