In a mobile-first index, Google has warned us that our primary content should be on the mobile version of our site, if we still want to retain our rankings when Google flips to the mobile-first index. I go way into detail on this over here.
But what if your mobile site is only your AMP page, that is one question we didn't cover because I thought it was a bit obvious. But nothing in our industry is obvious - maybe?
So if you have a mobile site and an AMP version, Google will canonical to your mobile site for the content. But if you are AMP only and that is your mobile site then make sure your AMP version checks all the boxes that your mobile version of your site would with the mobile-first index change.
Gary Illyes from Google said so on Twitter:
@AlanBleiweiss "same" centerpiece at least. But forget the AMP part, they are just mobile friendly pages in our index.— Gary Illyes (@methode) November 13, 2016
Then former co-worker and former spam fighter at Google said on Twitter that AMP is no replacement for a mobile site:
Repeat after me: AMP is not a replacement for a mobile friendly website.— Pedro Dias (@pedrodias) November 13, 2016
Then Pierre who is also a former Google chimed in saying it might be, in which John Mueller who is still at Google said it depends:
In any event, if you are using AMP for your mobile site and that is all you are using for mobile - make sure your content and links are in that version. Of course, there isn't much navigation when it comes to AMP, so it does scare me to have AMP as the only version of your mobile site.
Forum discussion at Twitter.
Update: Read this new story named Google: AMP & Desktop But No Mobile, Mobile-First Index Picks Desktop Version.