I don't see WordPress.org, or even WordPress.com, moving away from PHP. With PHP 7 just around the corner, most of the glaring performance issues are no longer issues. I do, however, think that we will see more client-focused work, such as themes and the admin UI, moving to REST-based implementations and abstracting away more of the PHP core code.

I was disappointed to see that this wasn't built to use WP REST API right away, but rather uses the WP.com API, though I understand why - WP REST API isn't in core yet. The Calypso team has publicly stated, though, that they expect to see parity between the two APIs in the future, which could make the Jetpack requirement go away completely. Either way, this is an exciting development and a very forward-thinking modern architecture glimpse at the future of WordPress!