I believe thinking about it as "offline vs online" obscures what's actually difficult about offline operation. It's not storage or ownership of data that's difficult, it's conflict resolution.

The longer you're disconnected, the more things diverge, the better your merging/resolution schemes/algorithms need to be.

There are a lot of ways of thinking about and approaching the problem, some of which work worse or better under different circumstances. You see it in distributed consensus, multi-master systems, version control systems, etc.

There's ongoing work in the form of things like operational transforms and CRDTs, but clearly their needs to be a lot more progress in the area.