JavaScript is not the only language available for the browser. ActionScript (Flash) is also available in many browsers, albeit fewer every day. Also, most browsers have a Java run-time for "applets."
Anyone sufficiently motivated and competent could write a browser plugin that holds a run-time for Python, Ruby, C#, whatever. The problem is getting people to install it or to get the browser makers to include it as a default. By historical fluke JavaScript got baked in to browsers early on.
The number of Flash-enabled browsers is not getting "fewer every day"; it's growing. The ratio, however, is probably falling, given the rise of mobile and decent alternatives for things like video.
Anyone sufficiently motivated and competent could write a browser plugin that holds a run-time for Python, Ruby, C#, whatever. The problem is getting people to install it or to get the browser makers to include it as a default. By historical fluke JavaScript got baked in to browsers early on.