The only code base that wound up being unmaintainable in that particular startup was the Clojure code base. I guess my point is no matter which language you are using, they all suffer from aging as they grow in size and addition of maintainers over time. The python code base was all incorporating mypy into the build / deploy pipelines and it worked quite well in my opinion. I am a huge fan of the recent additions to python and the work done there for static typing like mypy, pydantic, etc.