SXG and WebBundle basically help map that trick out into the browser's domain, rather than some nasty restrictive JS library. It allows eliminating the whole megabyte-sized supersystem of JS nonsense that makes up the AMP library and having just plain ol' regular HTML/JS, while still netting most of the performance benefits of the old approach.
Mobile networks may have improved but the speed of light has not, and so these techniques still have a place in contemporary networking.