I think you're overcomplicating how transferrable framework knowledge is. An expert React developer could switch to Vue no problem if there was demand for it. Similarly an expert Vue developer could switch to React no problem.
I am not. The original question was about starting a new project and the orinal answer was “depends on the project”. I am asking how it depends “on the project” and not on the person. What properties of the project would drive you to choose one over the other?