>keeping local devices in sync for all but a very few uses is already today easily solvable.

No, it really isn't. Manual file transfer is... doable, but still more problematic than it should be. But there is absolutely nothing that will seamlessly keep my phone in sync with my tablet when I don't have network access (i.e. I take a photo on my phone and it's on my tablet, without me doing anything). If you have a solution, link please.