I can give a production example of migrating to React.
We needed to do it incrementally from a server side rendered Java application, with jQuery and a lot of custom spaghetti code.
At first we migrated the server side templates to JSX and started to use React as a library by calling ReactDOM.renderToString. The first thing our JavaScript did was render the React templates similar to using Handlebars. Then the rest of our code kicked in using jQuery bindings and it’s events logic.
Now that our templates were all JSX. It was simple that start converting leaf “components” with simple state into ReactDOM.render instead. Often needing ReactDOM.unmountComponent to “refresh” the state And rerender everything like the jQuery based code needed.
Finally, we worked our way up the stack of components so that more and more were “just normal react components without calls to ReactDOM.
The last thing we did was migrate the top level to use ReactDOM.render and we were fully React based.
FWIW, we thought about Redux but didn’t use it. We used the patterns from Redux, but felt we didn’t need a library to manage our data. This was also pre-Hooks.
All in all took 1 dev on a team of 9, that all kept working on the codebase, about 4 months working on it off and on 50% time focused on the migration.
There were no issues caused along the way, the automated tests didn’t need to be changed (except for the new features - which were orthogonal to the React migration), and customers had a (unnoticeable) latency improvement comparing the metrics at the start and end of the migration.
The incremental use was a huge reason we picked React. Not only to migrate to React but also because we can do the inverse process when we want to migrate away.
P.S. We finished this migration Jan 2019. Since then the productivity of the team has noticeably increased. Even new hire “ramp up” has been reduced with them producing production code faster, on average, than pre-React. I’m sure these successes would be true for a migration to any modern UI tool e.g. Angular or Vue.
And I think most people do use React as a framework. (But I could be underestimating those who use it in the second way, of course).