I wonder if for groups up to a certain size, every client could hold all of the data for the group. I think clients do this anyway for caching purposes, but it would mean that as long as any client still had the data, the room wouldn't vanish.

It doesn't scale so well, but if there were an easy ability to (say) plug in an S3 bucket URL for offloading older media, then it might work for quite a while.