The definition of "opt-in" is questionable. Many publishers don't want AMP and would not have implemented it if it weren't fast becoming critical to their survival because they are overly dependent on Google, and Google is leveraging its market dominance to exert control over the ecosystem.

This was in response to FB doing Instant Articles which had much the same effect on publishers. They couldn't afford NOT to be on FB, but still lose in the end because publishers have zero leverage these days thanks to Aggregation Theory [1].

[1] https://stratechery.com/2015/aggregation-theory/