Javascript is being replaced by newer (hopefully better) Javascript. That's they only thing browser vendors can get in agreement.
There's plenty of other javascript compilers for plenty of languages.
There's plenty of other javascript compilers for plenty of languages.
Because you don't want to write your backend in Javascript and you prefer a different language. You don't have to know well only one language in your lifetime.
Libraries. Backend code can be quite different than frontend depending on your requisites.
E.g., there's no SQLAlchemy for Node.
HTML and CSS are quite different things, you can't escape that.
Right tool for the right job. That's why I don't think you should use css on the server! Or haskell on the client's browser!
I'm sure it's a perfectly valid option to put in the browser, though.