This has absolutely nothing to do with AMP. May I remind you that slow Internet connections are not a new phenomenon. Try browsing the web on a 9600 baud modem. The means for solving this problem (content negotiation, carefully crafted pages, CSS media selectors) have existed since the dawn of the web, and AMP is a terrible solution that carries the collateral damage of obscuring the URL/source of the original resource and hijacking the presentation of the page.