And as for your specific question: typechecked code doesn’t get to production with a type error; it won’t compile. There’s a common phrase, “left-shifting errors”. It means catching bugs as early in the development cycle as possible. In terms of things like developer time (and patience), it’s far more cost-effective to do so.

I worked on OpenStack back in the day: millions of lines of untyped Python.

Let’s say you’ve got an X509 certificate. You know you can probably pull the subject out of it - how? Were I using Java (for instance), the types would guide my IDE and make the whole thing discoverable. The prevalent wisdom at the time was that the repl was your friend. “Simply” instantiate an object in the repl then poke at it a bit.

And it’s not just that kind of usability barrier. “Where is this used?” is a fantastic IDE tool for rapid code comprehension. It’s essentially impossible to answer for a large Python codebase.

Don’t get me wrong: python is still a great go-to tool for glue and handy cli tools. For large software projects, the absence of type enforcement is a major impediment to navigation, comprehension and speed of iteration.

Not just domain logic. The implementation logic is often weird too. Cobol systems have crash/restart behaviour and other obscure semantics that often end up being used in anger; it’s like using exceptions for control flow, but exceedingly obscure and unfortunately (from what I’ve seen of production cobol) a “common trick” in lots of real-world deployments.