There's a risk of being a "Jack of all trades, master of none" in Software Engineering.
Companies pressure Engineers to be "Generalists" because it lowers salary rates and makes people more replaceable. But it's the Specialists (like Dan) that drive actual innovation.
Do you think companies really have a policy of intentionally pressuring engineers with those motivations in mind? That sounds a little conspiracy theory. In my personal experience what's really happening at your average widget maker is they don't need someone who can write database software or a UI library for browsers. They see that the front-end person can get her work done in 2 days a week, so they try to find something else for her to contribute to. Specialists like Dan don't work for the average company, and he wouldn't even be of much use to them either. And specialists like Dan wouldn't have a job if it weren't for the millions of generalists using his work.
EDIT: In another comment Dan himself mentions that Facebook tries to hire generalists.
I bet if you got fifty programmers (as a representative sample) in a room and asked them to define "T-shaped" you'd get a wide range of answers that generally reflect a T shape but don't really align on what the horizontal part should be.
Companies pressure Engineers to be "Generalists" because it lowers salary rates and makes people more replaceable. But it's the Specialists (like Dan) that drive actual innovation.