For linux only, lan only shared drive NFS is probably the easiest you’ll get, it’s made for that usecase.

If you want more of a dropbox/onedrive/google drive experience, Syncthing is really cool too, but that’s a whole other architecture qhere you have an actual copy on all machines.

I see that being said quite often.

Is there any actual proof of this or is it speculation?

In low density population areas, it seems to me that laying fiber would be cost prohibitive, but I’d like to be proven wrong.