But you loose so much flexibility. When you start working on a problem, you want your language should give you as little headache as it can with minimum rules and constraints. Then on iteration you can improve like having type annotation, unit tests etc.

You can write perfectly fine production level code in Python. It like lego block. Start with simple and then add on.

Adding compile type checks comes with its own demerits. I guess here Swift is trying to offer more tool chain on compiler level for model building rather than just being type safe.