Conspiracy theory: This change is dictated by the Google AMP team that wants to take over the world without us knowing
I was just about to write this but I don't necessarily think it's that far off.
With signed exchanges, AMP pages have the ability to hide the fact you're accessing content through Google [1]. In 2016 Google wrote about testing 'mobile-first indexing' because more people are using mobile devices than desktop browsers [2].
[1] https://developers.google.com/search/docs/guides/about-amp#a... [2] https://webmasters.googleblog.com/2016/11/mobile-first-index...
If Google can control the URL narrative (keeping users from bouncing off AMP pages) it's just one more ability for them to be MITM.