What you're describing is fast scripting in a dynamic language, which is genuinely useful - I agree it beats 'edit, compile, link, run' for exploration. But a Lisp REPL isn't 'dynamic language plus introspection'. A Lisp REPL is a persistent connection to a running process where the agent evaluates expressions against live state and can redefine code in place. python -c throws the process away every time; a REPL keeps it. The difference is the same as between sending one-off curl requests to reconstruct a session versus having an open SSH shell into the box. Imagine using a Playwright/Puppeteer session where you can navigate to a page and interactively palpate every DOM element, like playing a video game, directly from where the code is. Now imagine giving that power to the LLM - it doesn't need to restart, re-compile or even save anything - it just goes and explores, changing the program behavior on the fly.

The type-safety-plus-dynamism point you make is real and interesting (basically Clojure with Spec/Malli), but it's orthogonal to whether you're using a REPL or just shelling out snippets.