Re: Lets step back a bit
"So continual saving means a new file format designed for that."
ODF formats have "flat" versions, i.e. not zipped and compressed. I thought that maybe they were intended for use with versioning systems built on diffs - SCCS, git and anything in between - which would overcome this. But no, elements are sequence numbered and a small change near the start causes the rest of the elements to be renumbered when the file is saved. It's no more suited to that than MS Office formats.
Clearly it would be possible for a file format intended to be saved economically in versions. It would also enable remote saving by means of a proper client server protocol instead of relying on emulation of a file system, something that would help protect against ransomware.
Perhaps it's time to start thinking of a new, open format based on these lines.