> I think the site owner should be blamed if content the user needs is not either on the AMP page or accessible with an easily found link.

Hmm. If AMP participation were voluntary, I would agree, but given how necessary it is to SEO ranking, Google deserves a large portion of any blame for broken content - They do not get a free pass to enforce usage of a protocol, but dodge the consequences of its implementation (which will always involve friction and some breakage).