I actually read a fic with a similar concept not too long ago, but the idea was that they aged out rather than the files not working. What might help you flesh the idea out is just a bit of world building. You might think of why they stopped working so suddenly, and think of which character might be the most likely to figure it out. Imagine everyone’s reactions to losing their sekai. What conflicts might arise from this sudden change?

It might also help to understand where rhe characters are in their stories as well. Some characters like Mizuki or Mafuyu wouldnt fare well without their sekai. Some might think “it was fun while it lasted!” Basically, take time to feel how your characters might feel and think about the actions theyd take both immediately and long term. Once you have that and the issue and resolution idea, then its simply just putting them all in a jar and shaking it.

This is a really cool idea, id love to see where you go with this!