Hacker News Reader

Google Drive does this within Google Chrome, fwiw. I don't disagree with the spirit of your comment though.

I'd say Google and Algolia are the only companies with the resources and know-how in search that can even think of this. And probably even they shouldn't.

The result can give a much better UX if done properly. (If done poorly it's just frustrating of course...)

Just think of SPAs vs normal backend templated apps. The SPA can be much faster with poor network connection or when the data needed is large but unpredictable. Then the SPA can really shine, if routing is (re-)implemented well enough.

What about MX (machine experience)?

I would agree, if the opposite didn't make the page unusable. Infinite scrolling doesn't really work if you don't unload what's on the screen (at least it didn't the last time I tried it, in 2014).