Earlier quoted context omitted.
It's perfectly possible to know several languages in-depth. It just takes a lot of time and effort. Moreover, it gets easier with time and practice. Knowing N languages makes learning N+1 language just a bit easier. For me the turning point was around N == 10, since when learning a new language - and yes, in-depth, including idioms, stdlib, some external libraries, maybe some framework (if needed), and also some fact…
Structural typing wasn't around in mainstream languages before Go and TypeScript.
> try naming any "new" feature which was added to your favorite (EDIT: I meant mainstream - TIOBE top 10 or 20 - lang here!) language recently - and I'll show you that same feature implemented 10 or 20 years ago in another language(s).
This is a fun game and I'll answer for them on this one: OCaml (1996) has structural types.