I doubt that CDN is really that important. Maybe if your site serves content overseas then there is a difference but what about a website in a relatively small country that is visited mostly by locals? The CDN doesn't make big difference here.

If your server supports HTTP/2 and has a connection with a good bandwith then static resources can be loaded really fast without any CDN because all the resources can be loaded in parallel without waiting in a queue.

The real motivation for publishers to implement AMP is that they hope to get better position in search results.