The point is that seamless peer-to-peer hasn't happened, at the OS level. If Dropbox' NS servers go splooey, there goes your backup.

However if me and every family member want to share our own private files, internally across the vast Internet, without any 3rd-party interaction - i.e. really peer-to-peer, only the open-source/3rd-party-app route works, feasibly, at the moment.

Which is what I mean when I say that I think that OS designers are asleep at the wheel; or, perhaps, brain-dead. An OS which does point to point encryption across untrusted networks, successfully, and which wraps up the whole thing in a workable GUI provided by the OS vendor .. its just not there yet. I suppose soon, though.