There are dozens of open source programming languages. If one really just wants to create a language with improved concurrency support, he/she can just take one

There are dozens of open source programming languages. If one really just wants to create a language with improved concurrency support, he/she can just take one and modify it – this would work a lot better than relying on ready-made components/libraries which can't be modified. There are some mature languages with an active "modding" community. Haskell, for example.