I broadly agree. Plain text formats remain consistently accessible and flexible.

That said, arguably the dominant AI artifacts are binary images, video, audio.

The "preview panes for other kinds of tasks" will be a big deal. Humans tend prefer rich text editors (I think?). Also spreadsheets, page layouts, node graphs, timelines, hex editors, ...

So maybe the preview panes are actually editors themselves? Speaking personally, I enjoy closely coupled combinations of underlying plain text representations with task-specific UI on top.