I can certainly see the benefits of creating a theme using ReactJS and the WP REST API, however, using the WP Rest API to fetch the data for the theme doesn't quite make sense as the WP Rest API is self consuming (the end points live on the same server that the theme is served from) and I can save myself the round trip requests from the browser to the server and then back down to the browser again. Perhaps using a GraphQL wrapper with the WP Rest API could cut down the number of data requests?