That would be amazingly impractical. May as well say “what if all website were forced to be .txt files”.
Most website template frameworks (Bootstrap/Foundation) etc rely on Javascript for basic UI features. Imagine having to wait for the server to toggle a simple CSS class on your page any time the user wants to view a menu, togle a button, or view a popup/modal/lightbox/whatever.
Yikes, how Draconian. Id be fucking pissed if someone came in and forcibly open sourced a product I had invested millions in developing.