It isn't only about the data. When people visit AMP pages from Google Search, they aren't visiting your site. They are visiting a restricted shell of it that is hosted on google.com with a left-right swipe that takes users off of your site instead of deeper inside it. It restricts your markup and monetization, and is one of the largest threats to the WWW at the moment. The web is supposed to be decentralized. Also, mobile-first doesn't mean mobile-everywhere.