The world isn't that simple. Search engines know how fast their fetches are all of the time, but they only know how fast a javascript-heavy site is when the search engine crawls with javascript on -- which is rarely.
And by construction, AMP pages are easy to analyze without having to execute anything.
You perhaps underestimate the complexity and fidelity of the Google crawl, indexing, and ranking pipeline. Many intelligent people have spent many years trying to solve the sorts of problems you describe.
(disclaimer: I work at Google, and used to work on Search)
I totally get that Google has done a lot of work on these issues. It is still the case that no website that wants good ranks should depend on Google executing their javascript. Implementing AMP, on the other hand, means that you know you're going to get a good mobile speed score.
In no way was I dissing Google's crawl or crawl team. I'm just handing out conservative advice for website owners.
(disclaimer: blekko Founder/CTO. Nice to meet you.)
And by construction, AMP pages are easy to analyze without having to execute anything.