I'm glad we agree that adding type annotations to code is a good thing. You aren't arguing against that–you are just arguing implementation details. If I'm designing a 5 9s service tasked with processing gigabytes of data every unit time, I don't want to pay the runtime cost of checking the types for every mundane operation like measuring the length of a string.
It is evidently not a solved problem. Why would large financial services companies be working on these things we're talking about if that were the case?
Do you know what solves the problem of checking every type of every thing you want to use before you use it, where the checks don't incur any performance penalty at runtime? Static type checkers!
Modern compilers usually don't even require any explicit type declarations, as they can infer at assignment. So they provide more safety for less code than your examples.
