Unit tests are not an adequate replacement for compile-time checks performed by a compiler. The former is lax and expensive, the latter is thorough and inexpensive.
Sometimes python is appropriate for production code, weak type system or no. Sometimes it isn't. As a project gets larger, more complex, and more interconnected with other things, static type systems become more useful.
End of story, surely? Didn't we all know that already?
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.
