Bottom line reality is, if the existing team is so awesome with React, what do they need the author for? He should remove himself on the ethic of Cost Containment. And if the actual reality is true, and the existing team only knows enough to understand React represents a conceptual shift in how to design code and how to complain about it, let the author do his job and the team grows their skills.
If they are building quality stuff that works well. Then they may be right about fighting against new stuff.
I’m getting really tired of being handed 5-10 year projects written in an abandoned framework.
Now they absolutely should be learning new things. This is our lot in life. To use it or not is a different question.
Or they are just lazy.
Additionally, the scale of its adoption seems to me a guarantee that it will stick around a long time. There's already Preact, but if one day Facebook stopped developing React, I'm sure someone would find a way to keep all that code running.