how does this square with very senior people putting in a lot of effort to bolt fairly good type systems onto Python and JavaScript?

> business logic will move faster than whatever code you can write and fix, and exposing it as just "types" breaks the process for future programmers to extend your program.

I just don't understand how this is the case. Fields or methods or whatever are either there, or they are not. Type systems just expose that information. If you need to change the types later on, then change the types.

Example: Kotlin allows you to say "this field will never be null", and it also allows you to say "this field will either be null or not null". Java only allows the latter. If you want the latter in Kotlin, you can still just do that, and now you're able to communicate that (or the other option) to all of your callers.

Typed Python allows you to say "yeah this function returns Any, good luck!" and at least your callers know that. It also allows you to say "this function always returns a str".