I don't see why Google needs to have its own cache for AMP pages. If a publisher has its own fast CDN, why not just let it serve the AMP pages from its own domain?
The pre-rendering is one feature that requires a trust relationship between the host (to only serve valid AMP) and the platform. Otherwise the page may e.g. use a lot of CPU while in the background.
So you’re saying AMP doesn’t actually speed up pages at all, and it’s only sped up because you pre-render?