Some individual has an understanding of each of those areas. Why does one individual need to know the entire food chain? The discussion point is about each specific area.

Even for the COBOL based systems someone understands it well enough at the conceptual and business level. And when there's no one to the company migrates and modernizes.

You are conflating the subject matter expert who understands one area well and others not so well with a general newbie who understands nothing well