We don't normally develop the OS ourselves, so what language it was written in is irrelevant. Same goes for runtimes, libraries, etc. If we're not responsible for maintaining it, it should not matter what language it was written in. Naturally, the "same language on client and server" argument can only apply to code we develop and maintain.
What is the benefit of using the same language on the client and server? That the developer only needs to know one language?
The server-side code you write has to interact both with the client and the underlying OS. So you should consider how language X interacts with both client and OS, in addition to the intrinsic qualities of the language.
If we adopt a modular approach to software development, then I don't see any universal benefit to using the same language for all modules.
js might be a good server-side language, but choosing it because it works on the client is the wrong reason.