I agree, it seems like an editor based on direct AST manipulation could work pretty well with existing languages (I actually prototyped one a long time ago in university). Some languages like C with text-based preprocessors might cause some headaches but there are others that probably wouldn't be too bad.
There's pretty far you can go with text-based formats since you aren't obligated to display the file exactly as it's stored on disk (and many current IDEs do minor code folding things). For example embedded images can be displayed inline in the source code, but be stored as some loadImage() function on disk. You could even have some comments with base64 binary data if you really needed - at that point binary vs text is mostly a performance issue, but parsing is usually pretty fast so being text-based might still have an advantage because of better interoperability with source control etc.
There's pretty far you can go with text-based formats since you aren't obligated to display the file exactly as it's stored on disk (and many current IDEs do minor code folding things). For example embedded images can be displayed inline in the source code, but be stored as some loadImage() function on disk. You could even have some comments with base64 binary data if you really needed - at that point binary vs text is mostly a performance issue, but parsing is usually pretty fast so being text-based might still have an advantage because of better interoperability with source control etc.