I think maybe one other lesson, although I certainly agree with yours, and with the other commenters who talk about the unreliability of this particular method? This feels like an argument for using an editor that autosaves history. "Disk is cheap," as they say -- so what if your undo buffer for any given file goes back seven days, or a month? With a good interface to browse through the history?
I'm sure there's an emacs module for this.