Then eventually they add static types. Happened to Python, JavaScript, Ruby... I'm sure there are more.
Statically typed languages put the onus on the caller to transform the data into the shape(s) required.
Dynamically typed languages put the onus on the called to handle anything.
That is, in a dynamically typed environment your function has to defensively code for every possible type it could be handed.
It's not about that at all. Static types give you errors reliably at compile time instead of randomly at runtime, better documentation of what the code expects (people writing dynamically typed languages eventually resort to type comments), working IDE support, reliable refactoring and better code, all of which results in faster development.
The cost is a more complex language, occasionally difficult-to-write types, and very occasionally impossible-to-write types. But those are very very minor in comparison to the pros.
This has never been an issue in Elixir, because instead of a comment, you'd just improve the pattern matching in the function definition.
def blah(%{students: [%{firstname: firstname, lastname: lastname}|[]], count: cnt}) when is_int(cnt):
fullname = firstname <> lastname