The content needs to be encrypted with the "AMP provider"'s key too, so each of the providers needs to be explicitly supported. The webmaster might go through the trouble of supporting Google and Bing, but I doubt many would bother with any of lesser-known search engines, if those were to adopt AMP.
Maybe, maybe not, but those are still options that are agnostic of Google. This doesn't rope AMP into being any more dependent on Google than it was before.
