Won’t you have to write entire stacks of code from scratch for lack of prior art in CL?

While the open source library ecosystem for CL pales in comparison to Python's or JS's or Java's (or even Julia's, if you talk about certain domain specific stuff), it's not like there's nothing. It really depends on what you're doing, but many sorts of common tasks have a library to help. https://github.com/CodyReichert/awesome-cl Plus, it's straightforward to make use of libraries with a C API, there are a few slower ways to call out to Python libraries, and there's a whole CL implementation built on the JVM if you really need Java libraries...

Common misconception but no.

The models learned to code, the actual language is just a tiny adapter on top.