Even in systems that continously save your progress there is a close analogue to manual save: Named revisions. Bookmarks of points in history that are better than the surrounding ones - points where some milestone has been reached, and some amount of proofreading done. Where there are no duplicated paragraphs or major rough edges like that.
If the current document is Untitled, New File, No Name, etc and does not actually point to a file, then the ubiquitous Floppy icon should act as Save As. Once the file does exist, a simple Shift-click of the same floppy icon should also act as Save As. Same thing as Cmd-S/Cmd-Shfit-S. However, I'm unfamiliar with this modifier click save as from any app.
