Only if they are correct. I am pretty sure that in old code base it will not be correct unless automatically checked before commit.
So the argument is analogous to saying "how can I trust the types in this Java program are correct, if it has never been compiled".
Well, whether some "old code base" has them correct or not, just run the type checker and find out.
In my opinion optional type checking is not comparable to statically typed language guaranties. So if I do not need types I would use python. If I need/want type specification I would use statically typed language.