Josh Pollock on twitter regarding Matt's objections: There are 2 big assumptions here - 1) That continuing to iterate in plugin is better than iterating in core. 2) That the features of the API that are proposed for 4.5 merge are going to pose a problem for core.

I agree 100%. Matt has failed to make any coherent argument regarding the downsides of merging the API as-is. His post "Chicken and Egg" was incredibly thin compared to Ryan McCue's "Progressive Enhancement With the WordPress REST API".