Isn't that exactly the model of Gdrive/OneDrive/DropBox etc? Except they might not do any LAN transfer optimizations, but that is minor implementation detail
I don't think those come even close. Some blocking considerations:
- They are a bolted-on "magical part" of your filesystem. If you manage your files anywhere outside of the magical field of synchronization, they're invisible (e.g if your file is in the "Downloads" folder, but that folder is not synchronized by Dropbox, then you won't find it from any other device.)
- They are not distributed filesystems. They are a central filesystem (on their premises, outside your reach) and a smart replication strategy. You don't get a say on data sovereignty, and your data lives on rented disk space.
- Trying to operate on your files is dependent on which device you're using. If you're on a computer, you get first-class local files in first-class directory trees, that you can manage with the usual semantics, provided you have enough free space in the physical disk of that device. If you're on a phone, you download copies of files (and pray they get uploaded if you change them in phone), and almost no way to manage the directory tree aside from the application itself.
edit: apparently at least Dropbox has had LAN sync for nearly a decade: https://dropbox.tech/infrastructure/inside-lan-sync