An AMP document is required to contain:
<link rel="canonical" href="...">
pointing to the regular HTML document.Except that doesn't put a physical link in the document. Why should I have to dig into the source code of the page just to find the link to the normal page?
Some non-mainstream browsers (ELinks at least) make <link>s visible. This is occasionally useful, but most of the time it just wastes space. :-/
You might want to use this web extension to make automatic AMP → HTML redirects:
https://github.com/da2x/amp2html
Or you could use this bookmarklet:
javascript:window.location=document.head.querySelector('link[rel="canonical"]').href
What's wrong with using CSS media queries to have a responsive design? Or if you absolutely need a separate mobile site for some reason, at least give a link back to the full desktop version of the page..
I get that AMP is supposed to help optimize page loading for mobile devices, but for most sites I don't think it is necessary to go to the extremes of AMP in 2018. Most places in the world have decent enough internet speeds and mobile data allowances that pages do not need to be super tiny all the time.