“Always connected” meshes don’t scale, but fragmented meshes that use store-and-forward for some data are workable. I’d like to see someone do this with Nostr. The main issue is that you’d have to limit the size of data in transit, so rich media is probably not realistic.

Think FIDO protocol rather than the always-on internet we’re accustomed to.

A mesh of meshes may also be possible in theory.

Mesh networks don't scale beyond a few thousand nodes if the nodes move frequently. Nobody has found a way to make that work. You can push another order of magnitude if you give up on decentralisation.