PhD training teaches people how to not plateau, as (1) it teaches one how to focus on the unknowns (in that context, in the entire field, so one can dig into them; more easily applicable to one’s own knowledge) and (2) teaches one how to learn without professors / classes, which mostly consists of evaluating the quality of textbooks and/ or research papers in a field, and also how to skim at just the right level so as to learn what one needs to know.