People often create to fulfill a need. The need might be intrinsic -- the need to create something. But back to the point: Agreed, you described well the exist

People often create to fulfill a need. The need might be intrinsic -- the need to create something. But back to the point: Agreed, you described well the existing tension. At one end of the spectrum you've systems like LaTeX and DITA; on the opposite side, Markdown. I don't think Markdown should ever be extended. If anything, it should be encapsulated.