I think the rest API will really start to show for end users when you start seeing a lot of themes use it for headless WP. I wouldn't be surprised if WP admin starts using this as well (much more so than it does now). You'll start to see a lot more interactive parts of the site that don't require a page refresh, that refresh data automatically, that keep you very connected to the site.

I do have to disagree on the sentiment that end users should have never been exposed to the term "custom post type." I don't think it was helpful to use the term "post," as something like "custom content type" would be much more accurate and less confusing. However, I always try and educate clients on how things work behind the scene, as it can sometimes help them see why some tasks take as many hours as they do as well as help them know what I as the developer have to work with. Not all of them want those details, but I find the ones that do have much less stress and feel more pride in their site. :)