> you seem to imply a dichotomy between a 2D pipeline and a 3D pipeline
I don't think that was implied. One can certainly defend the view that a program rendering 2D objects to a 2D output without (normally) doing 3D transformations on them could limit itself to using a 2D API. Especially if the 2D API is supported by everything your target audience is caring about, while the alternative OpenGL has pretty dodgy support on average.
Whether this 2D API is implemented in HW by a 2D pipeline, a 3D pipeline, or a horde of spice-swigging OpenBSD developers warping spacetime with their giant brains is besides the point.
There are of course arguments for using OpenGL without alternative: simplicity; that the 2D API is too crufty; that shoveling wastage at the proverbial fan will finally create enough of a stink that the OpenGL situation under Linux is fixed for good.