I think it is possible to be a React.js or Vue.js expert within 3 years (they've only been around for 5 years), but expert knowledge with these libraries also requires some fundamental knowledge of Javascript and the DOM that I believe can be learned in parallel with these libraries.
I think generally you are correct, but these might be the exceptions to the rule.
You could be a vue expert in only a few months if you already know programming and web development. In my experience vue only adds a few things on top of javascript which are incredibly helpful but not complex to understand.
I think generally you are correct, but these might be the exceptions to the rule.