Focusing on a single technology never was a great idea though. Even already towards the end of the 90's D3D was the better choice on Windows, so one had to writ

Focusing on a single technology never was a great idea though. Even already towards the end of the 90's D3D was the better choice on Windows, so one had to write code against multiple 3D APIs anyway. This also gives a better perspective where the strengths and weaknesses of the different technologies are and it makes it easier to learn new APIs. But in the end, 3D APIs are just a utility to write applications (mostly games), not to build one's career upon. 3D APIs come and go and are (to some degree) also subject to fashion cycles, the underlying GPU hardware develops a lot more predictable than 3D APIs (e.g. especially Vulkan had pretty bad "mood swings" recently). Of course when focusing strictly on money and "career growth", going into game development is a pretty bad idea to begin with ;)