Earlier quoted context omitted.
Why is that the only way? Seems like they could easily have achieved the same result by significantly penalizing sites based on load time and number of external requests.
Because users want relevant search results much more than fast websites. Google already factors in a website's performance in their rankings, but weighing it too much over content relevance will make search results worse.
If visibility is influenced by AMP then Google benefits, users using Google services likely benefit, web developers suffer, users not using Google services to view the content continue to suffer (because companies will continue to maintain two versions of the website, a bloated version with 100 external tracking requests that will be shared on twitter/reddit/facebook/hn/etc, and an AMP version that will only appear on Googles services), and the internet as a whole suffer. Whereas if visibility is influenced by page speed+external requests then everyone would benefit.