>But this is also about providing a better user experience.

Breaking the behavior of my back button is one of the worst user experiences I get these days. Usually it makes me so angry that I just leave the site right away.

> Rather than waiting for the HTML to be downloaded when a link is clicked, most of the UI can be displayed while a smaller payload is downloaded.

Browsers can do that on their own since forever, given a carefully crafted HTML page. Of course there are use cases where that is not enough, needing some load-on-scroll mechanism or something like that, but it gets abused all the time for things a browser could handle so much better (with proper HTM of course) - blobs, newspaper articles etc.