From HTML WG Wiki
Recent Research on Online Tutorials and Documentation
Recent research finds that a multitude of Tutorials, Documentation, and books exist. Obsoleting longdesc would cause all of these to be obsolete wasting an outlandish amount of time, money, and effort, which was used to prepare them. This critical support base has taken a decade to build and would take another decade to rebuild with a different mechanism. Obsoleting longdesc in favor of a new mechanism would setback progress.
Obsoleting longdesc would cause confusion to all who encounter these tutorials and documentation as they will expect longdesc to be a technique as described in the tutorials. These types of materials have a way of living on.