You're correct - Google search results don't return AMP links on desktops, but that doesn't stop me from stumbling upon them.
If you come across an article you want to share from your mobile device, and it's an AMP link, it's the AMP URL that gets shared if you post it on sites like HN, FB or Reddit. If I'm browsing those pages from my desktop, clicking that link almost never redirects me to the "original" page, but loads the AMP page in the desktop browser. Sometimes getting around that is as easy as cutting out "/amp/" from the URL, other times it's a totally different URL and I'm stuck staring at a blank page for 30 seconds before it either loads, I just give up or I Google the headline/title and try to find the original page.
Forced AMP results on mobile devices also make it difficult to get to certain pages when I want them. Take Eater's 38 lists[1] that they put together. If I'm on my phone and want to find a restaurant from that list in a particular location (say if I'm out of the house and want restaurants near me), then the AMP result returns a page that doesn't include the map, only a list, which isn't very helpful. In order to get to the map, I either need to go to Eater.com and manually find it, or use something like Bing to search for it. I know that the purpose of AMP is to not load the map in an effort to increase speed, but in that instance the map is exactly what I want and AMP makes it harder to get to.
I'm not saying AMP doesn't have its benefits, but its inconveniences have outweighed them, in my experience.
[1]https://sf.eater.com/maps/best-restaurants-san-francisco-38