To be fair: you can definitely call JS from Elm/Gren using ports, which exposes the JS world as an async service you can call. This seperation means you lose none of the guarantees of the language, while being able to communicate with the outside world. Kernel code is completely unprotected, and would allow mutations, exceptions and side-effects in "Elm world".

The main problem with ports is that (1) it's an async API, which can be awkward for certain operations and (2) you cannot define a package that contains ports.