What you haven't realised is the difficult part is automatically resolving sync conflicts.
Well, it doesn't seem to be a problem in the solution I'm currently using. Mind you, sarcasm>I probably have no clue how it works, given that I implemented it/sarcasm>. But you go right on ahead and keep telling me it's a hard problem. Difficult for you, perhaps, but not hard; there are a finite number of possible solutions and it should not be difficult for a well-built system to solve. True, Git (which I used as the basis for my solution) doesn't do a very good job of this natively; it took some creative and well thought out commit and merge hooks to accomplish it, a good day's work, for sure.
You are correct, though, that there is no one-size-fits-all solution for what to sync and how to handle merging of whatever does eventually get synched. But, then, I never claimed that there was; my claim was that the transport part of the equation is, and has been for decades, solved. Can you argue that point?
Earlier, you said:
They are file formats. They are not methods of handing off open documents between different devices without first saving them somewhere. Completely different thing.
And I didn't disagree. I did, however, point out that the documents are, in fact, saved somewhere (e.g. a temporary file, at the very least), out of necessity. I also pointed out why this was necessary, e.g. if you at all care about data consistency and preventing work loss in case of loss of power or a software or system failure. Can it be done without a temporary file somewhere local? Sure, and without issue, as long as you never lose connectivity or power while working, and your software and system never crash. If you live in a perfect world, you are correct to say that a temp file provides no benefit; however, neither I, nor anyone else I know, live in such a world. When you're using an all that utilizes cloud storage and the app crashes or you close it while you happen to not have any connectivity, it is able to restore your work only because it stored it in a local temp file somewhere.
What application(s) are you involved in. I would like to avoid them.