It's probably possible to have something approaching true mastery over compilers or physics simulation or distributed databases, or anything that's a field instead of a framework. Asking for mastery over React instead of over UI engineering is a little like asking for deep knowledge of Torx screwdrivers - there are high level concepts from internationalisation, accessibility, QA, automated testing, and UX design that will still apply in a decade or two, even if we write all our UI code in Rust and steam lightweight VDOM diffs to the server or whatever.