just think of any obvious function where type is obvious. surely writing typing information in those cases is reddundant to the extent that it wouldnt even help with optimisation. but think about something like this:

   bool function1(x y z):
          bool function2(x y z)

immagine function2 besides returning true/false mutates x in some major way. this is a far bigger and more common problem than typing. a dynamic language with capabilities of runtime debugging is far better equiped to inspect code like this

also i am not saying that typing is worse than no type information. im saying that typing should be optional as far as the compiler is concerned and typing information should be like documentation that is useful to the compiler. common lisp can be an example of a language that is both dynamic and strongly typed (see SBCL) to the extent that you can implement a statically typed language (ala ML) in it