Technically, nothing. But in practice, if Emacs ran JavaScript, people would overwhelmingly use Emacs to browse the web, which would involve the normal step of

Technically, nothing. But in practice, if Emacs ran JavaScript, people would overwhelmingly use Emacs to browse the web, which would involve the normal step of running the scads of non-free JavaScript code which is present on seemingly every web page. RMS is not aiming to simply have a technical excuse to claim that something is not his fault, he wants people to actually get into a better situation than they are now in. Therefore, it would result in a practically bad result if Emacs were to be used in this way.