Then we should probably define it before asserting that it's "completely unintelligible", "fragile", or a "hard problem", right? Yes, full multi-way sync is not a simple problem. Most scenarios don't need this (and even ones that do tend to devolve into simple cases since single-actor conflicts are not that common).

The simplest case for "sync" is just "my stuff is in the cloud". Call it remote storage, since sync is ambiguous. Make the client dumb, put the data online, and most of the complexity evaporates. Of course, local storage with remote backup is also a reasonable solution that has different tradeoffs.

For me personally, I prefer the "remote storage" solution for most things. I greatly appreciate that my email is just "magically" everywhere I need it to be without me carrying a repository of email in my pocket. I love that all my important documents and photos are accessible everywhere even if I forget my phone.