I don't think anyone really argues that everything should be plain text, even if that's an easy shorthand. The real argument is "use the simplest, most open format possible."
Nobody is suggesting you go through all of your photos, transcribe your emotional reaction to each picture, and then delete the image. But, if you want to view those same photos when you're fifty years old, or seventy-five, you're better off storing them as a JPEG than a PSD, and you're better off storing them on a hard drive you have access to in addition to whatever cloud they're currently occupying.
"Write plain text" is a shorthand for "use open formats." Because so much of what this audience does is test-based, plain text is the most common format we use, from source code to journaling, but that message applies to pretty much anything: if you lock yourself into a proprietary format, or a proprietary editor, you will almost certainly lose data over the long term.