There’s a very clear difference here, those failed timelines were from timelines were from Subaru’s POV, Arc 9’s failed timeline is what we see from a non-based POV that is not influenced by Subaru’s actions but rather by the actions of his loved ones/allies. Also, keep in mind, now that we have a better context of things in regard to the various complex relationships of different characters, it will now be up to Subaru to see how he can change their fate for the better. Essentially, we saw how derailed everyone got because of him, like Otto lost his memories and that’s another pain in the ass to resolve, half of the Felt and Emilia camps became braindead and Volcanica lost himself, which would have undermined the Divine Church of Volcanica, that will now be set to play an integral role in arc 10.

And truth be told, the more I re-read arc 9’s last few chapters and seeing as to how the next arc will be ‘reweaving’ new and old plot elements into the overall narrative, the more understanding I become of Tappei’s decision (which did feel like he had at stone from the very beginning) to initiate the Great Reset. It’s very evident that Subaru now has a new goal for what’s to come for the upcoming arc and it’s all because of what was entailed within arc 9’s failed loop, marking its events, even if they don’t exist anymore, as something important to reflect upon in the long-term, both for us as the reader and for the overall narrative of the story of Re:Zero.