You can totally do that, but I write React for the entirety of an app because it means not having two separate development flows for different features. Making

You can totally do that, but I write React for the entirety of an app because it means not having two separate development flows for different features. Making my login page driven by HTML and postbacks, rather than a straightforward REST(-ish) API talked to by an SPA, makes it nontrivially more difficult to establish the kind of separation of concerns necessary to then build a mobile application out of it (and as it happens, React is pretty useful there with React Native).