It would be basically impossible for React to reach jQuery-like levels of adoption. The beauty of jQuery was that you could take an old, crufty pre-jQuery application, drop in a single script tag, and start writing nicer code immediately with no fuss (and refactoring old crufty code as you touched it).

You can't do that with React. I can't even imagine what it would take to migrate some 2010-era J2EE enterprise app to React, but it certainly wouldn't be just dropping in a script tag and writing your new code more cleanly.

Which means that React can only be the product of choice for new development moving forward; and even then, given how complex it is, only some subset of apps are going to bother with it. Nobody ever said "this application is too simple to bother with jQuery," because jQuery helped even simple apps, but there are legitimately huge classes of apps for which React is just massive overkill.