Why though? With the changes announced in this post (e.g. using the web packaging standard), loading an AMP page from Google's servers will have pretty much all the same security guarantees that loading it from the origin's servers would have.
And in terms of functionality, does the AMP web version have all the functionality of the normal site? Often this is something I see missing on AMP sites. A top-bar gives you the ability to get back to the full site.
I think it's the other way around - webpackages include Expires: headers for each resource that are signed by the origin, so the browser decides whether to trust it or not, or maybe to render it anyway but show a staleness indicator. But if you're using a CDN, the CDN implements caching on its own (hopefully following the origin server's instructions or the customer's instructions in a control panel, but no guarantee) and all headers are entirely controlled by the CDN.
> And in terms of functionality, does the AMP web version have all the functionality of the normal site?
On the technical side, this is now the origin's decision, not Google's - they can include whatever functionality they want or whatever links they want in the webpackage. It's not really different from having a mobile site that has a "View full site" link, or worse, doesn't have one and also misdetects your browser.
I think I agree that in practice it matters a lot what people choose to do, and whether Google adds restrictions for what sorts of webpackages count as "AMP" for the purpsoe of special treatment on the search results page.