Yeah, while I basically loathed AMP for all the control and monopolization issues, I do see what Google was trying to accomplish, at least at first.

Any front end dev has had to deal with the onslaught of asks from various marketing and sales teams: "Can you add this tag library?", "We need to integrate this affiliate broker!", etc. etc. And lots of devs would push back with stuff like "At this point we load 247 3rd party tags and JS libraries and it takes 53 seconds for our page to load, we have to stop this madness!" but the problem was that for any individual marketing team ask, the impact was small and of course that team had some KPIs to hit this quarter. It was basically a sort of Tragedy of the Commons situation.

So AMP came along and essentially gave front-end devs a technical reason why they couldn't add some shitty, slow, buggy affiliate broker JS library to the code base, so when marketing came with an ask, they could simply say "Sorry, not supported in AMP, and without AMP we get downranked in Google". AMP essentially became a technical hack to align short term incentives ("We need to add some marketing feature X!") with longer term goals of faster, lighter-weight pages.

Yep. I totally see why they did it. It’s a user focus, not developer focus. Users just want faster webpages. The end.

Why couldn't they just directly downrank pages based on their size or load time?

>without AMP we get downranked in Google

Whether a site used AMP did not affect ranking in Google.