I can imagine this simplifying the development of B2B SaaS tools that are primarily about presenting and interacting with data. The bit about broadcasting to a large number of clients is harder to see the value of in a Rails application when so many approaches work simply enough already.

As for replacing rich frontend rendering libraries in more complex apps - it seems dicey. Part of the virtue of React is that it simplifies the visual presentation of the frontend based on a constellation of data. Things like highlighted elements, hover states, and dragging items. It seems difficult to enable that level of polish while the backend is making dom-level changes based on the explicit data that you have to deliberately send over the network.