Thanks, this was very informative. I'm not a fan of AMP at all, but this helps me understand the reasoning a little bit better and why Google hosting the AMP cache is necessary for preserving privacy.
At its root, I think my objections to AMP boil down to a few things:
On a technical level:
1. It's buggy and weird on iOS.
2. I'm not convinced I care about a few seconds of loading time enough to justify the added complexity of making this kind of prefetching possible. Additionally, this seems like a stop-gap that will be rendered unnecessary by increasingly wide pipes for data.
On a philosophical level:
3. It gives Google way too much power over content.
4. I want the option to turn it off completely because of points [1] and [3], and because I fundamentally want to feel in control of my internet experience.
Edit: The point about SXG making AMP URLs less likely to get copy/pasted to other mediums is a key benefit I hadn't considered and will likely make avoiding AMP outside of Google search easier.