I'm not sure if it's a particularly convincing axiom here for heavy payloads of web-apps, particularly when we're talking about simply delivering content to cite a singular code base for why a web application that would benefit from AMP (which was the central conceit of the actual blog post we're all here discussing today) needs to be 15MB heavy.
Well formed markup and smart content compression could accomplish the same thing.
I guess you got your wires crossed, as AMP is not something I was talking about - my focus was specifically on PWAs and the fact that it's not a problem if their payloads are large.
But yes, AMP (read: glorified CDN) wouldn't solve much if you have 15MB to download each time you visit a site (aside: you wouldn't need to do this with PWAs because of caching).
Many reasons, but the top one in my opinion is having a single code base across all devices. I think it's a worthwhile goal, personally.