In reply to Weston Ruter.

It will be interesting to see what direction Mullenweg and the leads decide to take in the foundational task of architecting the new editing experience. Mullenweg made it clear in the State of the Word address that he would like to see Calypso or a similar interface replace wp-admin in the future. However, Calypso was not built using the Customize API, WordPress’ own single page application admin interface that plugins and themes already widely support. …“But I do believe the future of a great wp-admin experience is JavaScript – probably React, talking to APIs, super fast, and maybe even working offline”

As I've written before, the customizer is already a modern JavaScript single-page application. We can definitely iterate on the customizer UI to make use of React but this would be an incremental (2x) improvement over the current customizer, whereas the technical improvements of Calypso over the WP Admin are much greater (likely on an order of magnitude). In either case, however, I think that the future admin should be built on top of the Customize API so that every change can be previewed, drafted, batched with other changes, and scheduled. Building on top of the Customize API certainly doesn't exclude building on top of the REST API. In fact, as of 4.7 the two are designed to work together so that you can apply the customized state to REST API responses. And as demonstrated in Bridging the Customizer and the WP REST API, REST API schemas make it possible to automatically add new resources to be edited and previewed in the customizer for free.