My opinion on this one, for games authors anyway, was changed by reading an early 2000s piece by someone rebutting a lot of the noise Carmack was making on the topic, focusing on exactly this point: by DirectX 6, you got an API that was a suite which gave you, yes, the graphics, but also the sound, the input handling, media streaming for cutscenes and so on. OpenGL vs Direct3D was a sideshow at that point for most developers: it was "solves one part of their problem" vs "solves all of their problems". And no-one involved in OpenGL showed any sign of being interested in those other problems.