Most people don't remember that email used to work offline. You would go online to send your outbox and receive to your inbox and after five minutes or whatever it takes to sync thee immense 5kB you cut this expensive line again. Then you can read through stuff and reply offline.

Yes, nostr should at least optionally work like that and it makes a lot of sense to split the relay (store and forward) part from the client. A client shouldn't have to care about storage or networking for that matter.