Comparing the development methodology of Facebook - the company behind React - and this mindset is illuminating. Facebook famously rewrote PHP and added extensions like XHP rather than start their codebase from scratch. React follows a similar philosphy. It can be incrementally introduced into a codebase without radical rewrites.
These lessons are largely ignored in the open source community, where everyone will gladly tell you how your tech is all wrong. This is actually really easy to avoid. Just focus on solving the problems you have and only adopt technology that is relevant to you.