Log is certainly a blob of data, but the point is that it should be more granular, with clearer delineation of what are and what aren't conflicting changes: there will always be conflicting changes where no automation can really help.

For zip and other container-type files, you'd have log entries to the tune of "changed contained file foo.png: ...".

Operating systems would need to support some basic operations: container file operations like zip files, basic bitmap image editing, basic text document diffing, structured text diffing (XML, JSON, YAML...), etc...

Applications would provide OS-registered services (like MIME handlers are registered today) that can interpret and produce even more semantic events on top of the existing ones.

The environment could offer an interface during "syncing" when it detects a conflict to resolve it using one of the generic (or not) conflict resolution mechanisms (use local or remote version completely; use incremental delta if there is some generic semantic diff; app-provided capability if present on both sides).

Now, you are right that this can be implemented with this log being a regular file next to the file itself and completely user-space, but you will hit issues if you are not able to tie it nicely to things like fwrite/fsync and similar syscalls.

Obviously, for it to make sense, it needs to be widely accepted as the approach, which is what the local-first movement is trying to achieve with CRDTs.