Nothing in these languages prevents them from being event-based or non-blocking. All these languages have perfectly fine callback architectures. Javascript wasn

Nothing in these languages prevents them from being event-based or non-blocking. All these languages have perfectly fine callback architectures. Javascript wasn't magically designed to be event-based, its just that the DOM APIs (which have nothing to do with JavaScript) were written in an event-based way because it really couldn't have been done in any other way. Similarly, if you were to put Python or Ruby in the browser you would necessarily need them to have some sort of DOM APIs (or there wouldn't be very much they could do in the browser). Those DOM APIs would once again be written in an event-based way. The "big existing libraries of blocking code" would be inconsequential since they would for the most part have nothing to do with the kinds of things you are doing on the web. Who cares if theres some file reading code in Ruby that is blocking? You don't read files in browsers. Think of the opposite proposal: if I went and implemented these same libraries in the same blocking fashion in JavaScript, it wouldn't all of a sudden become a worse language for the browser.