Correction: It lets anyone cache your page, not just Google. And no "masquerading"; that's what the crypto is designed to prevent. Also it's not specific to AMP; you can use signed exchanges with any data served over HTTPS.
It's effectively just Google since it's not widely supported by browsers other than Chrome. There's also only one CA provider that can create the right certificate for SXG.
Or maybe you have some notable examples of SXG being used in a production non-AMP scenario?
The standard is brand new, and AMP was the motivating factor for its creation, so obviously the majority of existing use cases are AMP-related. That doesn't mean you couldn't go and implement a non-AMP use case in your own production site today.
One interesting use case for SXG is to allow decentralised and offline websites, since the site's data can be tied to a key/certificate/domain without having to be downloaded from a specific server. As an example, the IPFS project is already trialling the technology:
tying it to a domain name (which is the typical use of the URL) breaks the web though. i could understand if the key is used to show that the origin is a twitter account handle or something, but breaking the semantics of the domain by signing the content doesn't make any functional sense. Other than putting lipstick on a pig (AMP) of course
