Well, "modern" is an ill-defined concept. As far as I'm aware, structural typing is not really that common, is it? Besides OCaml and Scala, is there any relevant (used outside of academia) language that supports it?
The disadvantage of C++ templates is the structural type is implicit - you only know if the input object satisfies the type if you read the documentation, code, or can decipher the error message that occurs if it didn't.
Concepts would have fixed this, but we don't have concepts and maybe never will!
Typeclasses aren't structural typing, they are nominative typing, as typing is controlled by explicit declaration of relations between types and typeclasses, not inferred from structural properties.
