> If people keeps creating their own extensions of Markdown it's because there's a need
This is a general misconception about why people create. People don't always create to fill a need - in most cases, motivation to create is intrinsic rather than extrinsic.
As for having a comprehensive markup language - every product & creative avenue in every field in the world needs to contend with balancing complexity & expressiveness with accessibility & simplicity. Markdown errs heavily on the latter side for the writer but comes with natural trade-offs (e.g. it leans heavily on the complexity side for parser-developers) - that push-and-pull will always motivate folk to try & strive for the magical best of both worlds.
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.
This is a general misconception about why people create. People don't always create to fill a need - in most cases, motivation to create is intrinsic rather than extrinsic.
As for having a comprehensive markup language - every product & creative avenue in every field in the world needs to contend with balancing complexity & expressiveness with accessibility & simplicity. Markdown errs heavily on the latter side for the writer but comes with natural trade-offs (e.g. it leans heavily on the complexity side for parser-developers) - that push-and-pull will always motivate folk to try & strive for the magical best of both worlds.