As a Guile hacker, I also think that more programs should be extensibile via Guile. However, I don't think programs need to "understand" sexps in the way you're describing. Programs can continue to work as they have, taking in textual input and outputting textual output. What you get with Guile is the ability to use the program's API via the REPL so you can interact with and extend that program, much like Emacs.