I think React originally started with the opposite intent: a library where you can mount a component onto selected elements of the web page. The lock in only happened when React was used to develop SPAs, which effectively meant that React takes over the document root. With that came state management, and frameworks that managed the complexity of state were not far behind.

Indeed. I've gradually adapted a server rendered jquery and HTML site to react by making react render a component here and there in react and gradually convert the site. Works great.