You'd have to train people how to use versioning even if versioning was baked into the file system itself.

> The other thing is that the model has been the basis for software's approach, in a way that has too much inertia. For example, Word documents that are drafts are saved as individual files. Why? If the filesystem enabled it, programs could save deltas and have an interface to freeze a draft.

Those temporary files are there as a volatile backup of the running state of the program. It's not really the same thing as a "draft" so shouldn't be treated the same. That is unless you're literally talking about unsaved documents in which case I'm not really sure how you expect the file system to predict what you want to do with a file that even Word - you're exampled running application - doesn't yet know what the user intends.

So what you're asking for there doesn't really make a whole lot of sense. That is unless you're trying to describe an auto-save feature - in which case Word does have literally that feature already bundled as part of it's default set up. Of course you do still need to save the file once to at least nominate where to auto-save to. But saving your drafts is good practice anyway (particularly on Windows).